Well, I scanned ebay quick and found this auction:

http://cgi.ebay.com/Bicycle-Hub-Cart...#ht_2121wt_913

Okay, so that appears to be the style bearing I need. I can only imagine there are many different kinds, due to the fact I saw an auction listed for 20mm 6804 bearing, 20mm 6805, etc. I'll have to figure that out when I ever need to get bearings.

Question, though. How many of these are used? Is there only two of them used, or do you "fill up" the axle so they are spaced the entire way across the axle? If so, generally how many of these bearings are used?

Does anybody find they have a preference, in terms of these style bearings vs open bearings?