You might want to look at:
1. Don's racing wheels from Mark Warwashana. He's in Michigan. You can look up his email and phone in the Torque Roll under the suppliers section. These are archived on the NCLRA website.
2. Perfect Co. Streamlined wheels part number P67 and P65. Not sure how hard the rubber compound on these is. I think I got some over at Smith's and intend to give them a try.
3. I don't know if anyone has any stock of the Glenn Lee urethane wheels anymore. I wouldn't mind having a few more of those. Billy Hughes might know?
Added, per Bill's input below--
4. 1-1/4" Wheels and axles from Bill Bischoff. Contact info in the latest Torque Roll.
Depending on what event you're planning on using them in, you might find the more stunty balloon wheels will let you set it down and stick it better without as much shock to the gear/airframe.
Hopefully, other guys with sources will chime in here....
Dave