They're pretty easy to find. Amazon.com: GRAND PRIX 200 AMP HIGH OUTPUT ALTERNATOR 99 0 01 02 03: Automotive
https://www.dcpowerinc.com/fit/2003~...3-220-spx.html
220A High Output Alternator for Pontiac Grand Prix, 1999 - 2003 3.1L V6 (189c.i.) - 8243-5-220 | PowerBastards.com

Thats just the first 3 from a quick google search.

But I would start with your power source (battery). It will make your alt not have to work so hard.