Not sure where to buy used bikes, but probably the closest bike shops to you are
Intown Bicycles, Atlanta Pro Bikes &
Outback Bikes, all about 5 or 6 miles east of GT.
Fixed gears can be rough on your knees, so you may want to get a good month or so using the existing gearing to build up your leg strength and supporting muscle structure before going straight to fixie. Plus you'll be much stronger on the bike and you'll be able to get more proper gearing. If you go straight to fixie, you might be overgeared in a few weeks as you get stronger.