I've never ordered from them until today, but Superdroidrobotics lists quite a few pneumatics wheels in stock.
the six-inch is about 5 bucks and the ten inch is about 8 bucks.
http://www.superdroidrobots.com/shop...nd-shafts/153/
They also have tread/track sold by the foot for DIY. 4-inch wide is about 10 bucks a foot, 2 inch wide is $6.50.
http://www.superdroidrobots.com/shop...nd-treads/162/
If anyone here has experience with them, I'd be pleased to hear your comments.