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 Mask can be found near Shoal Night Club. At the entrance to Shoal Night Club in the Yucatan Casino, go up the escalator and through the long hallway, then turn right and jump over the counter and you’ll see the Ninja Mask on a mannequin in the corner.
Check out my other guide if you’re looking for the rest of the pieces to the Ninja Costume.