Heh, auto parts store or kmart/walmart will usually have those things. If you have an uncommon car, you may not be able to get one.
Why do you want one, though? Just paranoid or actually think people are stealing your gas? I had one (that was standard) on my car, and it was pretty silly to lock/unlock it. Just imagine how dumb you'll feel if the lock ever breaks and you have to leave your car on the side of the road forever... ;-)