We've ordered from VXB.com several times an been very pleased with the result.
Last year we used these on our turret:
http://www.vxb.com/page/bearings/PRO...Susan/Kit11280
Poke around their website. They have a very wide selection. I'd probably go a step or two up from the 400 lb one if you are supporting an arm: the cantilever and shock loads on them can be quite high.