Waterworks/Lamson's Gear Shower is a 2.2 gallon tank that attaches to a lowered vehicle window, and the gravity fed hose can be used to clean off boots, waders, etc. Even better is that some of the proceeds go to the Clean Angling Coalition and it's yours for just $49.95.
You could also buy any number of simple camp showers that will do the exact same thing and donate the money you save to the Clean Angling Coalition.
If you want something with a little more juice, the Zodi battery powered shower is only $28. Submerge the battery-operated pump in any water container and switch it on for full cleanup mode, the pump adds water pressure to the mix.
Hot Jugz makes a portable 2.2 gallon shower for $49.99 that gives you 2 minutes of pressurized flow with 15 pumps and up to 16 miniutes of shower time.
If you want something portable with more capacity and pressure you can spring for a portable sprayer from Big Kahuna.