Apart from the knight pieces, these are some other unlockable clothing that are available in the safe-house closet after completing certain requirements.

Unlockable	How to unlock
Champion's Jacket	Win a TIR episode (be the overall top player by the end).
Convicts	Kill 10 psychopaths.
Hockey Mask	Use every melee weapon on zombies.
Tattered Clothes	Kill 1,000 zombies using hand to hand.
TIR Helmet	Earn $1,000,000 in TIR Online.
TIR Outfit	Earn $5,000,000 in TIR Online.
Willemette Mall Costume	Rescue 50 survivors.