FYI, since it had been hard to find in the past, we recently discovered that many Tractor Supply locations stock Shell Rotella ELC coolant in the gallon full-strength (concentrate). Calling around here in Maine, most seem to have 6 gallons in stock. $14.99/gallon. Just give them the SKU (1079982) when you call the store to check inventory.
Shell Rotella ELC Antifreeze, 1 gal. - Tractor Supply Co. (http://www.tractorsupply.com/en/store/shell-rotella-elc-antifreeze-1-gal)
Both tractor supply and NAPA stock it here in Houston, same price.