costumes!!
I decided to make a list of all the locations of costumes I know about, feel free to contribute using the comments! (thanks Doctor X, Largo, Arbitor, Manquilor, Mephisto, Hoggle, and Daimoth)
Lepori (rabbit thing from Arindal) - Junko's place
Bunny - East town in southeast corner
Purple Tor - Hiding outside of Purple Tor Inn
Snail - Snail racing place in Phisneyland on TI
Guard dog - feral den
Fen costume - Town hall (where you choose race)
Cat - Hustus' house (northeast of town center)
Chicken - outside Castle Alliance
Sheep - southeast farms in northeast corner
Horse - Shroom (brambles)
Darshak - Ash Outpost (south of horde)