FastL, watch Ebay for wheels, none will be usable as is, most will probably need modification for your use. Never try to put an aircraft tire on a one piece wheel. Watch government surplus sales. You may have to buy a lot of stuff but it will be cheap.
Acft wrecking yard... try Alamo Aircraft in San Antonio. There is probably a similar place near Ogden Utah, Warner Robbins Georgia and any other government repair depot. New tires must be mounted on a wheel within 3 years of manufacture or they go to surplus. Wheels are time change items. Just because a part has "timed out" for an airplane does not make it unusable for your purposes. Tires and wheels are sized the same, a 4.4x18 tire fits on a 4.4x18 wheel, wheel will be a 10 inch. You will likely find a lot more 20x4.4 than 18, that one has a 12 inch wheel. See the pattern yet.
The search takes time.