There's a guy on e-bay that sells aluminum made carriages and for a good price. Not that I'm trying to endorse the product but he has these linear bearing/bushings mounted to the bottom of the moving plate and he said that he buys them at Motion Industries. I have been bugging him so often that I feel I should leave him alone and ask the question here. Sales dorks at Motion Ind. can barely tell you what time is it and they are of no help. If you go to e-bay and search cnc routers, he's there all the time. They look like a standard version of a cast iron pillow block and fit over a 5/8" shaft (2 per shaft). it could be just a simple bushing with one of these electrical pipe clamps placed around it.