Where can I buy a goose?
Can't speak for Atlanta, but up here (Canada) when I am looking for a meat that a supermarket doesn't carry (like goose), I go to the local butcher. My butcher brings in a very good selection, and otherwise usually knows of more sources. Also good places to check are local farmer's markets.

Edit: Adding to an old answer as I've come across another additional thought. Another place that might be worth checking is the farms themselves. It's becoming more common to have farms that sell directly to consumers. Around here there's quite a few. A quick Google for "goose farm yourlocation" might be a place to start.