Since I don't have an RO/DI setup, and Thornton water is like pure sewage, fertilizer, and chemicals, I get all mine from Elite Reef in Arvada. $1.25/gallon. They sell the 5g jugs w/ handle and ER logo for $15, but first fillup (fresh or salt) is free. I tried Walmart, and the 5g camping jugs were nice, but the same price, around $14!
So it was a no brainer, just buy them from ER. Fillups are 50cents/gal. for fresh, $1.25/salt as Drew mentioned. Lady at ER said they use Kent salt, not sure which one exactly though. They got a 400g storage tank, nizzzzze.