I use the water from those R/O machine you find at local supermarket when i didn't have r/o unit at home yet. They work well for me. It wasn't bad, 1.25 for 5 gallon.
Reverse Osmosis. Water gets forced through a membrane that seperates junk from the H2O and discards the bad water. Generally speaking, it works kind of like how our kidneys work.
Use distilled water, not spring water. If you look closely at the fine print on the jug of water, it will list all the processes that were used to purify the water.