I've been using Wagner's from Reid Supply for years now, they have a urethane wheel and don't flat spot (even under my Unisaw). Also, the total lock version locks up nice and tight with no wobble. IIRC, I think I've got 5 machines on them.
http://www.reidsupply.com/online-cat...ret=sku/WC-73/
I use the WC-73 & WC-75 part numbers.
Ed