You can have a total of 10 healing gourds by the end of the game. 1 you get at the start, and the other 9 you find throughout the game world. Here are their locations:
1.) In the Ashina Outskirts; outskirts wall - Gate path idolGo forward from this idol past the building immediately in front. There will be a small arena with a Samurai general named Naomiri Kawarada standing guard. Defeat him for the 1st gourd seed. Note that you can sneak behind him fairly easily for a backstab deathblow which will deplete one of his two health bars. Not necessary, but greatly helpful.
2.) Ashina outskirts; Outskirts wall - Stairway idol Move forward from this idol until you see an outdoor area sloped upwards with a staircase going up and a large building at the top of the staircase. The gourd seed is inside the building. To get inside however, you must defeat the miniboss at the top of the stairs, the chained ogre. He has 2 health bars, but his attacks have a bit of a windup to let you know when he'll do something. When you 1st go up to him he'll be chained to a wooden post that he'll break free from a few seconds after spotting you, however that gives you plenty of time to get a fair few early hits on him. You can use the flame arm prosthetic found in the Hirata estates to stun and burn him, then get a few number of hits. If you're really struggling, a cheesy strat to make him lose a health bar immediately is to have him break free, run far away to the idol or even further back until he loses aggro with you, then once his back is turned you can stealth up and backstab deathblow him. Either way, defeat him then grapple up to a hole in the building at the top of the staircase. The gourd seed is against the wall on the left side.
3.) Ashina outskirts; Ashina castle gate idolA bit further ahead in the game. After beating the 1st intended boss in the game, Gyoubu Masataka Oniwa, there will be an idol in his arena in front of the large wooden castle doors. From here, head in the opposite direction all the way across the battlefield. You will come up to a large staircase leading to a building near the ashine castle gate fortress. Before heading inside the building with a very tall NPC, go to the right of the entrance until you reach the end of the walkway, then go left up another outdoor staircase. Past the top of these steps is a memorial mob merchant selling a few important items, including the gourd seed for 1000 sen. Note that this merchant will unlock the memorial mob achievement if you haven't yet unlocked it. He'll also be selling the firecracker prosthetic for 500 sen if you haven't unlocked that either. That prosthetic is needed if you're planning on getting them all to unlock the all prosthetic tools achievement.
4.) Ashina castle; Ashina Castle idolOnce reaching Ashina castle and activating the first idol of the area which is immediately in front of you, go a few steps forward past the large wooden gate. You'll see a large staircase, but go right before the staircase and grapple onto the top of a wall, then grapple onto the building straight ahead. From here you'll reach the rooftops which is teeming with these small swift masked enemies who will throw shurikens at you, they're called nightjars. They can be a pain to deal with in numbers so I recommend taking this slow and steady; try not to aggro more than two at a time. All along these rooftops in specific locations will be smoke signals rising a few feet into the sky. The smoke signals will eventually lead you to an open window near the top of the tall main building of the castle. After climbing through the window there will be a treasure chest on the inside with the gourd seed, as well as a new idol to activate.
5.) Ashina castle; Ashina Castle idolStart from the same idol as before. This time head up the large staircase a few steps until you come across the first grapple point on the right side. Walk along these rooftops parallel to the staircase and after the first building you're walking on, get onto the next building moving forward and watch the right side of this building until you see a narrow raised alley with walls on three sides. Drop down into this alley and against the backwall will be a friendly npc you can talk to. He'll ask you to clear out the samurai from the area so he can escape. Kill all the low level mobs on the left side of his hiding alley, then speak to him again. He'll thank you then give you a memo telling you to follow the smoke trails all along the rooftops, but I already told you that one. :) Speak to him again to exhaust his dialogue, then he'll move to the dilapidated temple idol. He's sitting along the right side on a mat and will be selling a few useful items, including the gourd seed you're after, this time for 2000 sen.
6.) Sunken Valley; Under shrine valley idol.
Drop off the immediate ledge and walk forward along the pathway until reaching the cliff edge. Across from the abyss you will meet the first of a new enemy type; an all female group of gunners. The ones in this area are easy to take out though so jump off the cliff and grapple onto the other side, then kill the gunner. Be aware that you can be shot down mid air so time your jumps properly. Run past the little cave until you see the next grapple point hanging above the abyss. Grapple again to the next tree branch and go to the left side and jump up the ledge. Jump again and up the rock formation, with the white paint markings, and grab onto the ledge at the top. Run along this small cave and to the end of the path. You should see a grapple point on your right side. After grappling jump onto the next rock formation on the left side, the grab the ledge at the top. Here you should see a wooden tent. The gourd seed will be next to the tent on the right side near an opening on the rock wall.
7.) Senpou temple, Mount Kongo; Senpou temple idolYou should be very familiar with the game by now so I'm going to stop giving very detailed specific directions in favor of more generalized but still helpful ones; especially since this next seed is a long walk from the idol.
Head out the gate into the outdoors of Mount Kongo. Run up the small hill to the edge of the cliff. Grapple onto the tree branch below and run up the stairs up ahead. Run all the way up the stepped pathway to the very end until you reach a large wooden building. On the left side of this building there should be a beam you can grapple onto. Grapple up then head into the building through the small hole near the grapple point you just reached. From here walk to the end of the room down the stairs into a new room. This room will be filled with a few small low level monks as well as a big fat monk. Deal with them however you wish. In the same room will be a shrine with some creature praying in front of it. The gourd seed will be in the creatures lap, so grab it then be ready to kill it afterwards.
8.) Ashina Depths; Mibu Village idolThis area has tons of zombie villagers that will spawn infinitely, so I'll give you the quickest route to the seed from this idol. Starting at the idol you should see a shallow stream of water on the right side. Follow it until the stream grows into a river. Continue following the river until you reach a lake in the middle of the village. On the other side of the lake on the left side of it is a large tree with white leaves. Follow the left side bank of the lake until you reach the tree. The gourd seed will be one of the items sitting in front of the great tree.
9.) Fountain head Palace; Palace grounds idolTo even get to this idol you'll have to go through the entire fountainhead palace area to the great Sakura idol. From there, jump off the tree into the water and swim to the submerged building where only the roof is exposed. An old lady npc will be standing here (who you should have talked to earlier on in the level) telling you pretty much that to get to the palace you'll have to swim through a cave where the great carp lives. She tells you so you can open the door of the palace so she can get inside ( I won't tell you why she wants to get inside, kind of an interesting moment). Anyways, go into the water and dive down. Swim forward and a little to the left side near the rock wall that acts as a barrier for the level. Dive until you find a large cave entrance. Head inside and the great carp will start patrolling the area if he isn't already. Try to hide behind him, but not so close that he spots you. If he spots you use B to dash away. Try not to get swallowed by him, because that's an instakill. There are some items in this area but for the idol then gourd seed, just head forward to the end of the area until you reach an underwater building with a hole on the outside. Swim into the hole, then dive down into another hole on the floor of the building, swim out then float up until you reach land. The area you see should be a hill with rock walls on all sides, small sakura trees and shallow water running down the hill. Go up the hill and there should be a grapple point on the left side. Grapple up a few more times and you'll come to the palace grounds idol. Kill the enemies inside the building then open the door on the left side for the old lady (she'll appear when you reset the area after opening the door). Along the right side of the building opposite of the door will be a hallway leading to the next area. The chest will be on the right side of this hallway. This should be your last gourd seed.
Each time you get a gourd seed, give them to Lady Emma and she'll give you one more charge of your healing flask. For the 1st half of the game she's standing right outside the dilapidated temple. After a certain boss she'll move into Lord Kuro's room. If she's not in any of these locations she's likely either inside the dilapidated temple at one point in the game, or next to Lord Isshin Ashina in his tower. Either way she's never too hard to find. Giving her the last gourd seed will unlock the achievement.
Important notice To even get to the fountainhead palace and see the true ending and get to a few bosses and the last gourd seed, it is important that when you go to Ashina castle a second time while it is being invaded during the story and you go back to the top of Ashina castle, you will encounter your foster father named Owl previously thought dead. A cutscene will play out and Owl will then give you two choices, either
A.) Obey the Iron code and forsake Kuro, or
B.) Forsake the Iron code and stay loyal to Kuro.
CHOOSE OPTION BOption A will whiplash you to the end of the story prematurely, locking you out of the last area, as well as a few bosses and the last gourd seed. This option is not ideal for a first playthrough. Choose option B. You will now have to fight Owl then the story should continue on as normal.
Oh and also, here's a youtube video for all the gourd seeds if my directions are too confusing. (Not my video btw)