I know Distilled water can get $ but repairing or replacing parts can be $$$.

Tap water varies greatly and can be very hard (lots of minerals the leading death of misting systems)
Bottled water has minerals added can be or worse than tap, but doesn't have chorine.

rain water is good but also varies where you are located and when local conditions change. If you are in the mountains
and no industry for 100 of mi then you might be safe. but that can change.

The one and only safe choice IMO is distilled or RO
You can buy either a water distiller or a RO water filter both are $$$
and both require maintenance at some point in time and replacement parts.

The hassle free way is a trip to your local mega mart and buy a few gallon jugs.

depending on how much ventilation and heating you use and the time of year and the ambient RH%
will determine how often the mister will run if you have it on a controller that has a RH sensor. and not just a timer.
My self I use a fogger and the above is true for foggers as well if not more so.
Case in point,
I raised the temp in the enclosure from 80 to 84F and the fogger that is set to 75%rh
started to run every 5 to 10 min whereas before the temp change it only ran once an hour.