If you dig through the layers of paint on your stock wheels, you should find stamped part numbers and ratings. All that I have seen have been rated for 9.00 x 20 through 11.00 x 20 tires. The couple of old riveted wheels I have listed up to 12.00 x 20 tires (although I think you need spacers to run them as duals).
If you are buying surplus 11.00 x 20 tires, I'm surprised that they don't include the tubes and flaps, although usually with no guarantees . When I did mine (Goodyear G177's), all had tubes and flaps, all the flaps were fine (in much nicer shape the some of the ancient crusty 9.00 flaps in the old tires), all except one of the tubes were fine, although I had to replace a few of the valve cores. Note that if you are getting surplus ones from around here (or up near Red River Army Base), make sure you pull the old tubes and flaps and bail all the water out of the tires (from rain and outside storage)!
Since you are in Houston try Gensco
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They buy lots of the military surplus tires and wheels, and when they have the right size, they are cheap. They will even ship stuff from their main location (Alabama?) for free if you can wait.