Goofy's default shield obtained at the start of the game:
Knight's Shield
Bought at Huey, Dewey, and Louie's Item Shop in Traverse Town:
Stout Shield - 250 munny
Mythirl Shield - 750 munny
Golem Shield - 1000 munny
Onyx Shield - 2800 munny
Smasher - 250 munny
Gigas Fist - 1000 munny
Adamant Shield - 4000 munny
Olympus Colosseum:
Herc's Shield - Defeat Hercules in Hercules Cup 9th fight
Genji Shield - Defeat Yuffie in Hades Cup 5th fight
Save the King - Win Hades Cup Time Trial after completing Hades Cup normally and solo
Heartless Drop:
Defender - Dropped by Defender.
After defeating Riku a second time in Hollow Bastion farm the Hallway in Traverse Town, Wizards also spawn here so you can farm for Donald's dropped staff at the same time. The ability Encounter Plus will speed this up obtained after you complete 15 syntheses this ability causes Heartless to reset after just 1 area change instead of 2.
Chests:
Mighty Shield - Chest in World Terminus at the End of the World after the Neverland warp
Synthesis:
Seven Elements - Appears in the synthesis list after you've synthesized 30 unique items
Recipe:
Blazing Stone x3
Frost Stone x3
Lightning Stone x3
Dazzling Stone x5
Spell Arts:
Dream Shield - Speak to Merlin in the Mystical House after acquiring all spell arts
Spell Arts are dropped by White Mushroom heartless after all 3 spells in the charade mini game are the same.
Traverse Town, Red Room: Fire, Blizzard, and Thunder charades
Wonderland, Lotus Forest: Fire, Blizzard, and Thunder charades
Deep Jungle, Tree House and Camp: Fire, Blizzard, Thunder, and Cure charades
Agrabah, Treasure Room: Fire, Blizzard, Thunder and Gravity charades
Halloween Town, Graveyard: Fire, Blizzard, Thunder, and Aero charades
Atlantica, Undersea Cave and Sunken Ship (belowdecks): Fire, Blizzard, Thunder, and Stop charades
End of the World, Linked Worlds: Fire, Blizzard, Thunder, Cure, Gravity, Stop, and Aero charades
Encounter Plus does not affect the spawn rate of White Mushrooms.