The Horror 1 and Horror 2 Mags each give you a 25% boost to PP earned from zombie kills.
The BIKES Mag makes motorcycles 3x more durable which means your motorcycle won't break as often. Horror 2 Magazine = Fortune Park, on top of the scaffolding next to the main doors to the Fortune City Hotel. Horror 1 Magazine = Underground, In the Americana Casino's Underground Loading Dock, by some Construction Hat's, Folding Chair's and a working Cash Diet Slot Machine. Bikes Magazine = Underground near the Yucatan Casino Ramp, on some boxes in the left hand corner. That's 3 pieces of the Ninja Outfit, the location of the 4th piece will be given later and you'll understand why in a moment. Ninja Mask - Entrance to the Shoal Night Club in the Yucatan Casino next to cash register. Hop on the red telephone booth and make your way up to the secret room. Ninja Hood - Royal Flush Mall, 2nd floor. Ninja Footwear - The Man's Sport store, Uranus Zone. When you start Sandbox Mode I would get all the following: With the Ninja Outfit Frank can basically travel the entire map without being bothered by frequent Zombie attacks and makes Sandbox mode much easier while chasing achievements (although Psychos and survivors can and do still see Frank regardless). Sandbox mode is fantastic for this achievement and many others however I recommend acquiring the Ninja Outfit because it makes Zombies ignore Frank (Not 100% ignore him but close enough). If it were me and I was just starting this game I'd do Sandbox mode first (before Story mode) as the method to this achievement will easily level you up to the max of 50 and provide you with millions in cash. However, after having gone thru it I've developed some extra hints, suggestions and strategies that while they might not get you to 100,000 any quicker they should definitely help you out in the long run. All the solutions/suggestions above are excellent and helped me get this achievement.