Item #1: Find a tire shop owner with some ball$!
There's nothing wrong with sectional rims, if they're cleaned and inspected, then assembled properly. 8.25X20 tires are actually one of the smallest sizes of 20" tires out there, and your local farmers' Co-op store should have access to thousands of them. Be sure to get new inner tubes and "liners", the rubber flap that fits between the tube and the rim. They should be included with the tube-type bias-ply tires at no extra charge.
I just called my local Farmers' Co-op store, (Fayetteville, TN) and the Co-Op brand "Super Fleet" 8.25X20 tube-type tire costs $123.25, including the liner. The tubes cost $14.95 each. No problem mounting them! Ain't it great, living in backwoods Tennessee?
Jerry