What you will have to do is start calling the local post office.....tell them you know they auction or sell their used vehicles from time to time and wanted to purchase one of them. Although they may not sell the used vehicles, they may direct you to the main or regional post office. Usually they are the ones that either auction them off or sell them from time to time. If you see a post office that has a maintenance dept for the vehicles, then chances are that is where you will have to go to buy them. Hope this helps...
Fwed