I need a shapefile of the limits of the city of Atlanta, USA.
Check Mapzen's metro extracts for Atlanta. They use OSM data and I believe they've also imported Fulton County's data into it.
They also have different formats in case you need something other then shapefile
Fulton County has GIS data available. If you cannot access the data for download on their GIS website, give their GIS department a call/email and ask if they have an FTP site or other means to download the data.