Actually, if you are talking about M38 tires (or M38A1), they are single piece wheels, so even though you do have tubes to worry about, you don't need the cage for inflation and a "regular" shop could handle them. You do want to go to a shop that is used to working on truck or agricultural/equipment instead of a shop that deals only in car/light trucks as you want somebody that can handle tubes, older tire designs, and who isn't worried about selling you thier tires...
You can get them from somebody like Westech and pick them up a show (or get a buddy to snag them for you), order them from a MV dealer such as RAPCO, Midwest Military or a tire dealer such as Wallace Wade, or sometimes, you can get your local tire dealer (if they are halfway competant) to oder them for you.
The STA brand carried by several of the MV dealers are made by Specialty Tire of America which used to be known as McCreary, and the brand is carried by a lot of dealers accross the US, so getting them to order some shouldn't be too hard...
Good luck.
