Sorry, no info on the best Joann's or Walmart - I'm sure a local will come along with that info.
But just an FYI - since you're travelling such a distance - if you're going to be there in October
http://www.sewingexpo.com/GeorgiaQuiltShow.aspx (about 25 minutes from the airport).
And if you're interested in other fabric sources, here's a list of GA. quilt shops:
http://georgiaquiltcouncil.org/?page_id=16
And here's the Atlanta Quilt Shop Hop - there's a list of the 12 shops on the right hand side, so those would be the closest:
http://thegreateratlantaquiltshophop.blogspot.com/
Will you have an address in Atlanta?
I think on another thread I suggested taking advantage of online sales and prices, and shipping to a US address.
Have a great and safe trip.
(the only reason I have those links handy is that I was just this minute trying to decide if I have the time to go the show.)