I naturally assume that's how The Punisher be affording all his guns... he be shooting villains in the head then steal their fortune before blowing up their houses.
Yeah. That's how The Punisher does it. He's sitting on top of millions stashed in several safe house, across the city, in a couple of states, all that he's robbed from his victims. He also takes guns, vehicles, and whatever else he gets his hands on.
But these are the Ninja Turtles. They aren't "stealing", besides, they put a damn shell logo on everything.