How did Mills and Somerset know the leather shop they went to when they were trying to find the blonde girl in Se7en? How did they know they would find something there? How did they even manage to reach that shop from the first place?
They found a receipt for Wild Bill's Leather Shop in John Doe's apartment:
John Doe's apartment is freaky. It's all black and has all this shit from the murders, like the hand of the guy he cut off. Also, Mills finds pictures of himself in the guy's bathroom, which he converted into a darkroom. That means the photographer at the scene of that last murder was John Doe. See how bad he is fucking with them? Somerset finds all these notebooks with John Doe's thoughts and the problem is that he isn't too insane. His thoughts actually make sense….too bad he kills people huh? They find a receipt from some exotic leather store and they decide to check it out tomorrow.