I have the Dessertreaded 21x800-6 tires and love them. I just bought two more of them for my Mangy Fox project, $109 each with free shipping.
There is now about 400 hours on the ones on my yellow plane with little if any wear on the treaded surface. I chose these because I operate off a paved airport but they still have plenty of diameter and soft enough sidewalls for the off airport playing that I do.