A basic 24 gpd R/O unit will recoup the cost of going to the store to buy water --very-- rapidly (I reclaimed the cost of the unit and started saving money after ~3-4 months with 1 gallon per day worth of usage)... But seeing that you only need a couple gallons, most supermarkets (Kroger/Giant Eagle) and department stores (K-mart/Target/ect) carry purified water. Usually the label on the jug will discose the purification method.
Another thing to check, and I'm not necessarily recommending it, but the water cooler at my workplace also dispenses R/O water (check with your manager first, of course!)
If you know anyone in the car wash biz, R/O water is used (just make sure it doesn't have any potential exposure to soaps)
just some ideas.