Is the bearing about the size of the ones in a 40mm grenade? I've never taken one apart to find out, but maybe they could work. I've seen the hopup rubbers around but I can't seem to find one right now. Maybe airsoftparts or ehobby or something like that.
Check out this link. I believe I see a ball bearing in the bottom left of the picture.
http://www.airsoftparts.ca/store2/in...oducts_id=1577
Hope that helps.