I know that it's years late, but he could have bought a Maynard/Namath one and had the name/numbers stripped and replaced. Would have been an expensive way to get just the shell of the jersey, but it would have worked.
Sweet I need to make my Favre jersey something useful, probably a personalized jersey with my name on it. (All the other #4s in the Reebok/current jersey era were punters). Nice I knew Cosby's did this, but didn't know for how much.