Check out
http://andymark.biz/products.htm
They have two varieties of 6" and 8" omni-wheels (aluminum and plastic). They also carry 4", 5", & 6" Traction wheels. The traction wheels are built to use dead axles, but AM also carries a "Universal Hub" with a 1/2" bore and 1/8" keyway. You could either modify your shafts to 1/2", build your own hubs, or modify their hubs to fit your shafts. Depending on your robot design, they shouldn't be too much of a problem.