I would say getting the 1$ jugs at walmart would be the best bet, although that can really add up as the time goes by depending on how much you use.
I don't own a mistking but I do buy distilled water frequently for my 20 gallon salt-water tank. There are some nice RO systems avalible at most saltwater fish stores online if you are interested in one, although they are about the same price as a mistking, maybe more.