I often find myself phrasing questions like that. 'You wouldn't happen to have any stamps, would you?' 'I don't suppose you sell beans?' On paper they may seem rude, especially the last one, but in person they come out more self-deprecating. Like 'This is a silly question, I'm well aware that the answer is probably no, but I'm looking for a violin, do you have one?' It suggests that if the shop owner/librarian/person who you want something from hasn't got what you want then that's entirely expected so it's totally fine and not their fault, but if they do happen to have it, it would totally make your day.