Land and Sea Exotic Pets is just outside of Powder Springs. I've never been there, so can't attest to it's quality, but they have stellar reviews and DO currently have live rock in stock. Might be worth checking out, if it's closer to you.
if You're looking for Tonga Branch you're probably going to have to get it private party. If they're willing to sell it, I know I'm not willing to sell mine LOL