I have bought bearings from both:
Skate Bearings:
http://www.skatebearings.com/
and
VXB Bearings:
http://www.vxb.com/
I have been happy with the quality of the bearings from both companies, however, these are import bearings not American made. All of the bearings on our robot this year (except for the ones that came with the AM gearboxes and IFI wheels) came from Skate Bearings and held up just fine. Bearing prices have really seemed to jump in the last few months. You can get really good deals on Ebay as well.
VXB sells on Ebay cheaper than thier website if you buy like 10 of the same bearing.
This Ebay store also seems to have a great selection and good prices.