You will find the costumes in the room where you use the boutique key.

Costume	How to unlock
Biker Costume	Complete the game under the hard difficulty setting with a rank of A.
Disco	Complete the game under the hard difficulty setting with a rank of A.
Jill's RE 1 costume	Complete the game under the easy difficulty setting with any rank better than F.
Police Miniskirt	Complete the game under the hard difficulty setting with a rank of A.
Regina's costume from Dino Crisis	Complete the game under the easy difficulty setting with any rank better than F.