I'm not quite sure why you have a hard time imagining this. The comics have showed over the years that he's bought, rented, or just taken over various apartments, storage facilities, abandoned maintenance shacks, and warehouses. Often if it's one he frequently uses, he'll set traps in it and if he does pay it's usually by cash under a fake name.Out of curiosity, how does Frank have stashes and safehouses when he works alone?