Distilled water from the local drugstore is not identical with water that is offered in pharmacies. Distilled water that is sold at drugstores is usually only decalcified, not sterilized. Pharmacy water is also sterilized. If you have no opportunity to buy sterilized water, decalcified water is better than just water from the main. Like many cigar smokers we simply use the cheap decalcified water with some of my personal humidors and never had any problems with mould. Depending on the worth and quality of your cigars, nevertheless, I recommend to use sterilized water. A good cigar needs something nice to drink while aging. J If you have a distiller for decoration purposes, you could also take it and make some sterilized water yourself. Please remember, that the kind of water you fill in your Humidifier just sanitized your Humidifier. The cigars rolled by a normal human person, touched by a lot of hands, packed in a wooden box, opened and loaded in your humidor with your hands already contain germs and bacilli. (Cigars from the shop sometimes are already infected with mould, which you cannot see, because it just grows inside the cigar. Placing such a cigar in your humidor can infect your other cigars.) The same applies to the food we eat. Everything we see and touch is contaminated with germs.
However, the whole issue is not a main problem as which it appears on some sites on the internet: In order to prevent germs from mass spawn, which means mould, check your humidor regularly. The humidity should always be at a level of ca. 60-70% and the temperature should not be higher than 20°C. If you, additionally, clean your hands before touching your cigars, you will probably have no problems at anytime.
If you would like to buy anything to reduce the risk of mould, I recommend an exact Digital Hygrometer: The humidity is not always identical at any place and in each corner of your humidor.
Alternatively, you can simply boil water and let it drain off. Please never use water from the tumble dryer, because of possible.
If you use liquids, please make sure that the humidifers don't become to dry, because some fluids could drain off accidentally, when the H2O contained is gone and only the thicker parts of the liquid remain.
We recommend using distilled water as not all customers have clean water from the tap as is available in the UK and Germany. It is important to use water that is as germ-free as possible. For example, you can simply boil water yourself if you are unsure. A low lime content is important to avoid blockages.
Distilled water from the pharmacy is the safest recommendation worldwide.