A quick Google search on "CO2 supplies Charlotte, NC" gave these results. I think there are more, but I just copied & pasted these:
Machine & Welding Supply Co -
Machine & Welding Supply Company
518 West 24th Street, Charlotte - (704) 376-7334
Directions, hours, and more »
National Welders Supply Co Inc: Sales and Service -
www.airgas.com
5421 Old Dowd Road, Charlotte - (704) 392-7317
1 review, directions, and more »
High Tech Garden Supply HTGsupply -
High Tech Garden Supply
2712 Freedom Drive, Charlotte - (704) 697-0911
Directions, hours, and more »
I don't use CO2 so I don't have any personal experience with trying to obtain it. I'd call your local place first and see what they say.