http://oddsandendos.safeshopper.com/4/50.htm?731

Some have been asking where to find these bearings. Price is great plus shipping is free.

Please note, the bearings require only minimal axial pretension. If you adjust the bearing off the bike, you will exceed maximum allowable axial pretension and this is what causes failure. In other words, the bearing adjustment will be sloppy off the bike. There is nothing you can do about it, save replacing the axle with steel.

I believe this is why my bearings have not had a single problem.