You could buy an RO
unit, you can find these pretty cheap on e-bay or check some of the sponsors of this site for sales.
Option 2...look around your area for one of those kiosk water machines. They're usually outside grocery stores or wal-marts even beverage distributors have them. If you find one, look into how the water is filtered, usually it is RO
and or DI
along with UV
, carbon and microfiltration. The machine water is fine for smaller tanks and is what I've used for the last year.
Gallons of distilled water from the grocery store is also an option, although without knowing how the water was distilled you could be introducing copper or other anwanted elements into your tank.
I think treated tapwater should be your last option. Tapwater CAN contain nitrates, phosphates, copper, chloramines etc. If you are going FO
then you could probably get away with treated tapwater. If you are planning on a reef tank then I would definitely hunt down a purer water source. HTH