Under the post "Knifemaker supplier list", check out what these guys have to offer. These would include Texas Knifemakers, K&G, Koval, Jantz, Tru-Grit, Sheffields, etc.
Other sources for contact wheels are the places that carry Burr-King, Wilton, etc. such as W.W. Graingers.
When it comes to contact wheels expect to pay around $200-$250 for a quality wheel from these places for either an 8 or 10 inch wheel. While making a grinder for hollow grinding, the contact wheel and the motor are the most expensive items.
C Wilkins