This kicks ass:
Pump-up pesticide lawn sprayer (clean) $9 at Wal-Mart
3/8OD, 1/4" ID tubing
Mister with plastic ballcock valve $5 at Wal-Mart
1/8" to 1/4" tubing adapter, found at Autozone in a "vacuum tubing" assortment pkg $2
Take off the sprayer handle (too big to mount on the headstock, and the handle would be hard to turn on and off) and tubing (it's too short), heat the end of the new tubing (as long as you need it to be) and press onto the fitting, join with the plastic mister tubing with the adapter. Use the little stopcock valve.
Figure out a way to attach nozzle to the headstock.
Now I don't have to worry about plumbing water in or getting a pump.
That'll hold at least a couple of gallons, even with a lot of space left for air pressure so the pressure won't drop too badly as it empties.
I started with one of those "personal misters". It was nice at first but had under 5 minutes of spraying capacity before the pressure drops noticeably if you're not pumping, even if you only fill it halfway so there's plenty of space to pump with air. That garden sprayer will probably have like a good hour's worth at least. I can get a bigger one if I really need it.