You can get 6 gallon water containers from pepboys, autozone, etc. You'll find them right along with the gasoline containers. If I'm using ONR, I bring my own water. From a 6 gallon container, I can usually do about 2-3 washes. Of course that depends on a few factors such as the amount of dirt, size of vehicle, if I accidentally kick the water container over while the cap is off (don't ask)

, etc.