Even if you live in an area with very hard water you can soften it by vigorously boiling tapwater for 5 minutes and allowing it to stand overnight. The temporary hardness (calcium ions) will precipitate out. The permanent hardness (magnessium ios) have no effect on photographic uses. The next day decant or filter off of the clear portion for use in developers.
I would think the cost of heating, boiling water would exceed the cost of just buying distilled water, unless it just not available where you live.