To access some new costumes, beat the game getting the 'good' ending. When you start a new game, you'll have the Four Leaf Clover Key in your inventory. Use it to access the room of the same name to get some new duds.
Octopus and Crab costume:
Find the October Ticket during the Once Upon a Time chapter.
Rag Doll and Stuffed Brown costume:
Find 4 Broken Robots during the Rag Princess Sews chapter.
Brown as a chair:
Once you're in the Four Leaf Clover room, inspect the chair.
Thanks to Revolution Reader Carbunkl!