the rainwater idea is a good idea, if u could harvest it neatly and reliably.
A double di resin vessel set up can be quite good, and could be set up for a couple of hundred new. If it was only used 4 times a year or so, it wouldn't run out too quickly, esp if the slow rate through the resin was slow,
Be aware that if there is a large build up of water marks, you might need to use MDR (mineral deposit remover)
Otherwise u might find they look awful still even after you've cleaned them with pure water.