Depends on the year... but for most of them, you can go to the auto parts store and get one for 80-120 dollars.
I had a 92 seville with 4.9 engine, the alternator was REAL easy to change, 10 minutes maybe. If you have a northstar, it might be harder. Just call around to all the local shops and ask!
If you cant do it yourself or find a mechanic, you will have to "shop around" local places... most will charge 300-400 for an alternator replacement.... depends on how long it takes to change out.