There are two Horror magazines in Dead Rising 2 for the PS3, Xbox 360, and PC that will each give you a 25% PP boost when killing zombies.
Check out the full Dead Rising 2 store map if you need help finding them.
Horror 1 – Ragazines (R212) on the second floor in Royal Flush Plaza
Horror 2 – Stan’s Large Print Books (P206) and Magazines on the second floor in Palisades Mall