Have you tried one of those thrust bearings. I was just looking through Tower to see if I could buy one like the one that came with my Slapmaster kit. I'm at work and can't measure it but since it goes on a 1/4" axle, the ID should be about 6.3 mm right?
This one looks close:
http://www2.towerhobbies.com/cgi-bin...?&I=LXCDV0&P=7
and then the next size up I see is 8mm.