I need some help with finding a decent website to purchase performance Mustang parts...?
Try summitracing.com or jegs.com
A few others are americanmuscle.com, latemodelrestoration.com, lightningforceperformance.com and so on. I could offer a lot more, but I don't know what year of car he has. All the above sites have been great to deal with for us.
If nothing else, run by Wal-Mart or anywhere that carries magazines, and pick up a copy of Muscle Mustangs and Fast Fords. There are lots of vendors in there that advertise, and you can check out each one's site on the computer to help make your decision.