Once you've got it apart & the bearings out, take them to your local industrial park and find a bearing shop (check the yellow pages). They can match them up with an equal quality bearing for a couple of dollars.

By the way, are you certain it's a bearing? I ask because it's not at all unusual for the rubber mounts for the alternator to break down and tilt the alternator so it's misaligned with the belt. Spray a little water on the belt and see if the noise goes away... if so it's likely those bushings. Less than $2 apiece, I think, from FCP Groton.
-Chris