Ben, there used to be a "quarry" in N Ga, somewhere around Dalton I think, that had salt, lime, etc by the truck load... been out of area 20 years, so don't know if its still there, and don't know purity of salt!
Outside of that, Sams or Walmart 40 lb bags is the best option I know- $5 bucks a bag, for the Solar Salt here in VA (so about $1200; you probably can find someone to make a delivery cheaper)