You might try Colson wheels. A quick search found
this site, where model WC18 (6"dia x 2"W) sells for $7.50 each.
Colsons weigh a little more than AM or IFI traction wheels, and their CoF is a little less -- but the price is right and they are easy to drill, saw, machine, etc. to suit your needs.
The site above and similar sites that offer robot components will have other types of wheels, too.
__________________
Richard Wallace
Mentor since 2011 for FRC 3620 Average Joes (St. Joseph, Michigan)
Mentor 2002-10 for FRC 931 Perpetual Chaos (St. Louis, Missouri)
since 2003
I believe in intuition and inspiration. Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. It is, strictly speaking, a real factor in scientific research.
(Cosmic Religion : With Other Opinions and Aphorisms (1931) by Albert Einstein, p. 97)