I found a great deal at a local heavy truck tire shop, although mine are 22.5" tubeless I think you should be able to find some used 20". 9.00x20, 10.00x20 were very common on all medium and heavy trucks.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy