There are a couple in that area. Marine Designs is in Suwanee. PureReef and Warehouse Aquarium are in Alpharetta, and Aviarium is in Gwinnett. Not sure how close to Flowery Branch you are, but Chris's shop FishScales is there as well. All are ARC Sponsors.
But there are many others with great fish around Atlanta as well. Don't limit yourself to just one or two based on proximity. You'll miss out on a lot of fun visiting other shops.