There are a number of places to get it filled.... I used to use Pye-Barker (on Roswell Rd) for my homebrew stuff when I lived in Marietta. Any place that fills for fire extinguishers or homebrew stuff will work. CO2 is CO2.
Also, be aware that sometimes the tank has to be hydro tested if older then 4 years (for a small fee) and swapping tanks may be a good idea and aluminium is better then steel around marine setups.
Jakup.... yes, that is a beast.