You're not going to be replacing the bearing in the compressor, its the one in the idler pulley, and that shouldn't have anything to do with which compressor you have. As was mentioned, bearings are sold by size and the parts store will probably just measure it and find you one that matches.

If you have a bad bearing in the compressor, you mostly likely will have to replace the compressor, which also involves evacuating and recharging the system, etc. $$$

Another possibility for noise is that the compressor clutch is rubbing instead of fully disengaging.

Compressor noise should disappear when the A/C is off (and whenever the compressor is not running as it cycles on and off).
Idler pulley spins all the time, so should make noise pretty much all the time if its bad.

Clutch noise could be either one. If its not disengaging fully, it would make noise when disengaged. If its slipping when it shouldn't, then it would make noise when it was supposed to be engaged.

And then there is the obvious that you already mentioned- slipping belt, which would only occur when the compressor was running also.