Hi ahatten, welcome to the forum. I found tires for my 2 ton at a tire shop that specializes in heavy duty truck and school bus service. I bought 4 virgin Firestone's that came off of school buses, I paid $60 each plus mounting and some wheel refinishing, I also bought 4 Michelin RV tires that I found on Craigslist, these were like new and I paid $75 each. Your 20 inch tires are going to be a little more difficult to find, mine are 22.5 inch and are still being used on modern trucks. edit-I just saw your pictures, 22.5 inch are widely available, make some phone calls locally for school bus garage, big truck service, etc.
