One piece wheels, two piece wheels, or double beadlock wheels?
One piece 20's- Mainly only used in Ag. applications. With as many hill jacks as there are in OK, there is surely an implement yard somewhere. Anything with floatation trailer tires like a spreader or fertilizer typically used the one piece 20's.
Two piece 20's- Used on nearly every old 18 wheeler. Find an old truck tire store and they'll pay you to take them.
Double Beadlock 20's- Standard equip. on a revamped 5 ton military truck. Surplus yards, gov. liquidation internet sales, and ebay are the best places to look.