I imagine that the TARDIS would have a room of lost things. Everything in the Universe that gets lost ends up in that room and whenever the Doctor is in need of something that he does not have he can just enter that room and search for it. Whenever he finds something particularly cool he removes it form the room to add to his collection. You know he found something in this room when he says he doesn’t remember where he got something. It is just a huge room filled with piles of other people’s memories, aka stuff.