It's going to be a lot cheaper than online ordering, even if you include the gas. Under the Wave and The Fish Store are fish only places, and very, very high quality. They are worth visiting just to see their stock. You could combine the trip with a visit to Atlanta's new Aquarium, biggest in the world, to make the trip incredible. They have a giant coral reef tank!
I imagine Under the Wave's and The Fish Store and More's price per pound is higher than Petland's. I have bought most of mine at Petland, although my biggest chunk, a great pillar in the center of my tank is from Under the Wave. Both rock hauls were very good, with the Petland rock containing many sesile worms, feather dusters, and a vermatid. The Under the Wave rock has a burrowing clam. Petland and The Fish Store have bigger quantities at any one time, Petland having several hundred pounds in it's own special tank, while The Fish Store keeps theirs in several regular tanks (full). Under the Wave sells the stone from their fish tanks.
The Fish Store is on Peachtree Street just after you pass the Buckhead park, where Peachtree and Roswell Roads meet, on the right, in a brick strip center. They are closed Mondays for tank maintenance. Petland is a chain, but I'm only familiar with their Norcross store. It is in a big strip mall with Home Depot and Target off from SR 141 (Peachtree Parkway) about 25 minutes north of The Fish Store (with good traffic). Under the Wave is harder to find, in a non-descript storefront 5 minutes east of Petland. Look them up and get driving directions from them, as I am really bad with road names! Good luck, and have fun!