You can use normal tap water, the ultrasonic discs in the mist generators have a short life whatever the water used, but they seem to last a bit longer if you use de-ionised or distilled water. If your water is hard you will find lime scale deposits on the disc of the unit and also the walls of the vivarium, using de-ionised, reverse osmosis purified or distilled avoids this. If you use hard water you will need to clean the glass with vinegar occasionally.
Just make sure the water is clean not stagnant or green with algae to avoid an unhealthy aerosol which could damage your health (it will not do the newts any harm!)