I just got off the phone with the guy at the hydro store, he said that they only have a bottle of 500mL for something like $18... I asked him if they had anything cheaper, he said no, so I asked him if there are alternatives. He said you can just cut a little square off a sponge, keep it moist, and keep that in the cap... He said, "that's what we do..."
He said they even use tap? If I were to do that I'd use distilled, but would THAT work? Seems logical, keeping it moist but not saturated with nothing that has impurities, right?