Remember these 7" smooth wheels from Mecoa.
http://www.mecoa.com/cb/wheels.htm They have some bounce to them which makes for soft landings. And the price is right. I have yet to see an 8" tire. I have looked until I am blue in the face for larger wheels but have had no success. I have seen site that they taken hand truck wheels and shaved them bald but I think that is a bit to heavy a wheel for my use, and they would be hard with no bounce.