It seem expensive at first but the RO/DI is best in the long run. Hauling jugs gets old quick.
You are looking for RO/DI water too not just RO water. RO still pass plenty that you don't want in your tank. That and supermarkets use lower grade membranes to make drinking water than what we would use for reefkeeping.
You best bet is are the LFS like AE, Teskies, or ROE.