To get new costumes in the game,start a new game with either character on Normal.
Do not pick anything up.

When you get to the police station you will see some stairs. Go down the stairs and there will be a zombie.

Kill him and he'll drop a ''Special Key''.

Use the key in the lockers in the dark room and there will be a screen that comes up that asks you if you want to switch costumes.

Say yes and you will have the ''new costumes''