They sell the baby water at Walmart by gallon its like $1.45 its in the baby department
If you have flouride in your tap water (most towns do of you have city water, if you have a well than you don't have fluoride) then you should use nursery water. Other than that tap water if fine just make sure its cold when you add it to the bottle and then warm the mixture together. Don't add warm water from the tap to the formula
U can also boil tap water if u can't make it to the store, ide recommend jugs of water though- or bottle water for on the go..most safest. You don't HAVE to get the one in the baby section you can get the one from the grocery section as well.
I use Brita filtered water that water taste better than bottled water.
The main thing is purified water, you can get it with added fluorides or without. The doctor told me to do some with the fluorides and most without. The fluorides are OK in smaller doses when they are younger. You may notice a lot of loose poops if you use from your tap water just switch to bottled and you should see a difference.