Fish heads, Charlotte area
I'm having a hard time thinking of anywhere that might be convenient to Lake Wylie. Your best bet would be one of the larger Asian markets like Super G or Grand Asia, both quite far.
The fish mongers catering to the general public probably don't usually offer fish heads but I'm sure they would if you called first. Clean Catch and Deep Sea Market are well regarded by CH'ers.
I can tell you I've noticed the larger Harris Teeters carrying whole red snapper, if that's of use to you. Small heads of course but readily available. In my experience heads have to be very fresh to want to eat them, which once you get this far inland means scant availability or higher price. Good luck.