There typically aren't any used ones for sale, as they can be removed and installed in a new game. You will have to bite the bullet and either buy a game with one installed or buy new. You'll be waiting for a long time to get that one rare "FS ColorDMD" ad
Besides, they are the one rare mod that gets 99.5%-100% return, unlike all the other mods that exist out there. So if you did come across a used one, you'll be paying almost full price for it anyway. Might as well buy new at that point.