Colson makes a lot of wheels in addition to the "Performa" line sold cheaply at RobotMarketplace.
Colson has a line of polyurethane-tread wheels molded over aluminum, polyolefin, and cast iron cores. You can see the specs for these wheels here:
http://www.colsoncaster.com/products...e/default.aspx
I found some 6" polyurethane on polyolefin wheels on eBay for about $10 apiece.
I can't comment on the construction and concentricity of these wheels as I haven't received them yet, but if you ask me again in a few weeks I should have an answer for you.