Tap water that has been boiled and then cooled (some people leave the water in the kettle and pour it into the bottle when needed or some pour it into a sealed container like a thermos, some people let it cool in the kettle and then fill all their bottles for the day...) When you're making the bottle for babes you can microwave a mug or measuring cup with tap water in it for 1 minute and then take it out and put the prepared baby bottle into the hot mug to heat up the bottle (the preboiled water with the formula, shaken) it's not a good idea to microwave the actual baby bottle. We sometimes use bottled water if we don't have cooled boiled water ready when we need it or if we're out and need to make a bottle unexpectedly.
Boiling the water won't hurt anything. Boil it, let it cool and put it in a pitcher or something. I do mine like that and my baby has been fine. I did start out buying the baby water from Walmart but even though it's only a buck, it sure does add up.
I get Gerber water. It's purified drinking water, no added fluoride. 84 cents per gallon. I give it to the daycare for her bottles too so they don't use tap.
Re: Baby water