Howdy,

Here is a very good page on Bosch alternator tear down:

http://www.rangerovers.net/repairdetails/electrical/boschaltrepair.htm

Old Bearings
Rear-small: 6203 RS or 6203 2RS (rubber seal) 17mm ID x 40mm OD x 12mm W
Front-large: 6303 RS or 6303 2RS (rubber seal) 17mm ID x 47mm OD x 14mm W
Seals: Labyrinth closure system that keeps the dirt out and the grease in.

NOTE
Do Not use 6203 ZZ or 6303 ZZ bearings, these are metal shielded bearings, but are not sealed.
Grease will quickly fly out of it, and you'll have a noisy alternator in no time.

Your best bet for a positive identification is to tear it down, remove the bearings, and take them to a bearing supplier or a good auto parts house.

You can find a bearing supply warehouse in the phone book. I have dealt with a few different ones for work. They don't mind small quantities as long as you know what you need.

--
Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 52 year old fat man. ;-)