http://www.qbcbearings.com
They have some miniature ball bearings (in the catalogues)
http://www.bocabearings.com/main1.as...25&w1=.500&t=I
That's a large list of inch bearings they carry. The 1/2" bores are near the bottom with ODs from .750 and up.
I found these by doing a quick google search for "miniature ball bearings." Most companies do not have prices in their catalogue, such as those two pages, but if you call the company you can usually get a quote for the items. Sometimes they can even help you find a bearing if you have specific requirements. Hope this helps some.
By the way, what is your intended use for these bearings? During the season I used some 5/16 by 1/2" miniature ball bearings and they became one heck of a problem because my parts put too much stress on the bearings. Usually as the bearings become smaller and thinner the tolerances become smaller and will need to be in more controlled environments.
_Alex