It's easy but time consuming: Just play survival mode for as long as you can. Every now and then, a gallery scene will show up (as a random collectable item). If there's other easier ways I really don't know them.
There is an easy way, its one of the maps where like theres a car you can destroy... Its been a while since i played, On that map a guy who sells things will pop out i forgot when exactly he comes out but i think you need to fight certain amount of enemies first....
