Before you purchase a new alternator, determine what's bad.

Disconnect the battery negative, and unscrew the brush/regulator assembly off the rear of the alternator (has a T03-cased transistor). I would also remove the tension from the belt.

Inspect the brushes (are they worn down), AND the copper slip-rings that contact the brushes (looks inside the hole where the regulator assembly was).

Look for any wear or scouring marks. I use a small piece of 3M "blue" scotch-brite (not emery cloth that leaves deposits that can rust) to see if the copper slip-rings clean-up. If not, then you definitely need to rebuild/purchase a rebuilt one.

If it only needs new brushes, you can get the brush/regulator assembly for about $20-40 from this forum's sponsor: FastLane. Call them, it is not listed.

If your alternator is making a howling noise, it most likely needs new bearings.
You can either take it out, and bring it to a good alternator/starter shop (the ones that cater to the big diesel trucks usually are the best), and they will rebuild yours in a day.

OR, purchase a new/rebuilt one from this forum's sponsor: FastLane.

FWIW, I purchased a used one from a junkyard $60, and had it rebuilt $65 with a new brush/regulator assembly.

:-) neil