In Dead Rising 2: Off The Record for the PC, PS3, and Xbox 360, the Ninja Costume can make you quite a bit more stealthy. While wearing any part of the costume, you will be less detectable by regular zombies.
The Ninja Uniform is not found, but unlocked. To gain the Ninja Uniform, you just need to unlock the Elite Killer achievement / trophy by killing 1,337 zombies.
Check out my other guide if you’re looking for the rest of the pieces to the Ninja Costume.