So, I don't know if this is a national deal, but my local big name pet store (the one that doesn't have the dollar a gallon sale) had RO systems on sale for $100.
Also, you should be able to use just RO water for your mister. RO should remove most of the salts and whatnot that would likely clog the misting system.
As for rainwater, the pH is questionable. 4 is probably an average, it is definitely worth checking your own. Personally, I use tap water for everything, but I also have a well and live in the boonies. However, it definitely wouldn't work in a misting system.
~Alex