Good old Britta filter, but it really depends on what quality of water you have in your area. If your water TDS (total dissolved solids) is low then theres no need to filter just use straight from the tap, if it's high then use a filter or if you want buy bottled water.
It really depends on the water in your area. I don't want my chameleon to get fluoride and our water is now fluoridated. So, I bought a little distiller and that's working well. Reverse Osmosis filtered water is often available in bulk from "Water Stops" and similar places (I know one laundry that sells it). Getting a 5 or 8 gallon container and filling occassionally that will very quickly be faster than buying it by the gallon at the store.
For years I have used tap water that has been left in jugs overnight so the chlorine can dissipate. Unless the water in your area is not good, this should be fine. (Some of the bottled and treated waters have no minerals left in them.)