The costumes are found in the room where you use the boutique key.

Unlock Police Miniskirt:
Complete the game under the hard difficulty setting with a rank of A

Unlock Biker Costume:
Complete the game under the Hard difficulty setting with a rank of A

Unlock Disco Costume:
Complete the game under the hard difficulty setting with a rank of A

Unlock Regina's Costume from Dino Crisis:
Complete the game under the easy difficulty setting with any rank better than F

Unlock Jill's Resident Evil 1 Costume:
Complete the game under the easy difficulty setting with any rank better than F