Assuming you want to grind this out and have progressed past 'Harvest the Jungle' you can go to X 445.6 Y 728.9 and find about 5-6 plants mostly green -this is just southeast of Amanaki Town in the beginning of the game. There is also a small hill you can run off of to hurt yourself slightly. -If you need to refresh them save and quit and reload the game -THIS WILL spawn you back at Amanaki with the plants having respawned.Side info: Syringes have a variety of effects from healing you and pacifying/calming timid animals to letting you breathe for an extended amount of time. You can first craft syringes in the beginning of the game after the Harvest the Jungle quest.
This is a beneficial syringe to become aquainted that will heal you using the
button. It requires One green leaf.
However you cannot spam/use them if you are not injured. The easiest way to injure yourself is by using a vehicle and ramming into a tree and reversing and repeating the process as necessary SAVE FIRST. Alternatively you can repeatedly step into a small body of fire such as a lit trash can like those that are sometimes in Outposts.
Other options open up for recipes as you do more quests such as Hunters Instinct and Animal Repellent which require Amber leaves (a LOT more common) and you CAN stack/spam these ones by hitting 
and Navigating to Crafting>'Hunting' (under syringes).