PM Cameron.
I bought a 10lb from him a couple weeks ago and when we were chatting I think he mentioned he had another.
I get Co2 from Specialty Gases Southeast in Johns Creek, right off of McGinnis Ferry and 141/Peachtree Parkway. They do an exchange type thing, so no worries about buying a used tank as you won't have it long.
I haven't had to fill the 10lb yet, but I have a 5lb on my planted tank and it costs about $11 to exchange. Supposedly it's cheaper pound-for-pound the bigger you go, so 10lb should be about $16 or so.