Gary-
A great place to get spare parts is other kits. You might only need the tires, but you will get the entire rest of the kit for your stash of spare parts. Also look on ebay in the model section for "junk yard". There are always several auctions for built models and or parts, usually with something in common, i.e. muscle cars, drag cars, etc. I've never looked for off-road junk yards, but I'll bet you could find one sooner or later. If you can get to a big model contest, they usually have a swap meet. You can find lots of out-of-production kits and even some junk yard-type sales.