Where do you find the mushrooms in Kingdom Hearts?

Mushrooms may refer to either (a.) An item that must be collected at the start of the first Kingdom Hearts, (b.) White Mushrooms, (c.) Black Fungus, or (d.) Rare Truffles.

(a.) If you're referring to the item that Kairi asks you to find, simply check by the bridge, behind all of the boulders and in the Secret Place.

(b.) If you're referring to White Mushrooms, then here is a chart to locate them:

Traverse Town Hotel - Red Room

Wonderland - Lotus Forest

Deep Jungle - Outside of Tent

Deep Jungle - Jungle Hideout

Atlantica - Undersea Cave

Atlantica - The Shipwreck, Below deck

Agrabah - Treasure Room

Halloween Town - Graveyard

End of the World - Room Before 'Last Rest' Save Point

(c.) If you're referring to Black Fungus, then here is a chart to locate them:

Halloween Town - Pumpkin Patch

End of the World - Room Before 'Last Rest' Save Point

Agrabah - Bazaar

Hallow Bastion - Dungeon

(d.) If you're referring to Rare Truffles*, then here is a chart to locate them:

Neverland - Ship Deck

Monstro - Throat

End of the World - Room before 'Last Rest' Save Point

*Note: Rare Truffles are extremely rare.