FWIW Specialty Tires America makes the 8.25 x 20 and 10 x 20 sizes. I would guess the 10 x 20 is also still available from other manufacturers. I run a set of STA tires on a '47 Power Wagon and they have been fine... not a lot of miles though.

http://www.stausaonline.com/tires/sta-super-transport-2/