Looks like you can get a variety including "hubless" version at Fairlane products:
http://www.fairlaneproducts.com/tech...=4-Durasoft-R#
To buy them, they link you to Fixtureworks.net
http://www.fixtureworks.net/Home/Ord...C0%7C0%7C6%7C1
Lots of different variants, volumes, and prices.
We bought some from McMaster last year. Got about 1/2 of the hubs out OK, and half destroyed. This hubless and the 2.5" options might be nice.
Originally we were looking at Bane-bots, but ran out of stock, and the ones that worked well did not have good longevity.