Try this one. I myself use this one and like it alot. The only thing i do not like about them is they reduce the water pressure coming out the hose
http://www.autogeek.net/deluxe-filter-system.html
And i have seen this one
http://www.griotsgarage.com/product/car+care/car+washing/car+cleaning+tools/in-line+water+softener+%26+deionizer+with+fittings.do