I am newish to reining but I have never heard of Thomas Karr. I have found in the Atlanta area, you are going to have to drive a couple of hours to get to a reining trainer.
I ride with Francis Mercier who is west of Atlanta in Roopville. I have been riding with him for a year, and the progress with my horse has been amazing. Luckily, I am closer to him now but was driving 2+ hours every week to ride with him before he moved to his current location.
http://www.mercierperformancehorses.com/
He moved to this facility earlier this year: http://www.westgaridingacademy.com/
Heather Johnson is also good. I have had some lessons with her and have friends who ride with her. I also have friends who ride with Herm Sherwin in Watkinsville.