My generac uses two very common bearings:
6205 on the drive end (RS seals preferred because it's out in the breeze).
99502H on the tail. (RS seals not as important, but still a good idea).
6205s are used in all kinds of things - snowmobile chaincases, riding mower decks, ATV wheel bearings, etc. Very common and quite cheap. 99502H (funny number is because it's an inch measurement bearing) is used in a number of things as well - weed wackers, snowmobile bogies, lawnmowers, edgers, etc. Also common, also cheap.
Accordingly, I got them from these people:
http://www.mfgsupply.com/m/c/index.htmlWho also happened to have a really good price on the funky special throttle cable I needed for my 1988 Honda mower.
There are bearing suppliers all over everywhere. If you need a big industrial bearing, like you probably have in your ST head, I'd start off by looking for a nearby bearing supply house. There are several national ones as well. SKF, FAG, NSK, NTN, and Timken are good names. The Peers I took out were in pretty good shape (although I beat the front one up pretty badly getting it off the shaft, which had rusted oversize).
The speeds and loads on any gen head are very modest compared to some other uses of bearings this size. If you buy good ones, you should get decades of wear out of them with a once/decade regreasing, because the load is constant and shaft speed is low.
George's major complaint with the bearings supplied with ST heads is not the bearings as much as the grease they're packed with. Repack with a good synthetic grease and bob's your uncle, although the finish quality of them is not as good as a name-brand cartridge bearing.
And on preview - concur with Doug - installation technique is everything, and unless you had the option of being very, very nice to the bearings you removed, throw them in the trash. That's where the old ones from my genhead are going once the new ones arrive.