My favorite source of plant water is rain water (I have a rain barrel).
A cheap filter you can use is a Brita.
Apparently (according to aquarium owners I know) letting water sit overnight
gets rid of chlorine but not chloride. Fish stores can sell special
chemicals to get rid of both, but I have no idea what these chemicals do to
plants.
De-humidifiers are a great source for water without salt or chlorine.
Heather