Well, a couple of thoughts here. Don't take offense at my suggestions.
Has the alternator been tested?
Has the regulator been tested?
There are a few chain stores that won't sell you parts for anything over a 1 ton truck. (Autozone...cough.....cough....) What store are you trying to get parts from?
If you are sure the alternator is bad, there are some places that will allow you to send in the old one, they will rebuild it and then they'll send the exact one back to you.