I was considering that idea, collecting from the downspouts on my gutters.
What kind of pressurized sprayer would you use with the rainwater you've
collected in barrels?

Sure, search the web for suggestions on how to set one up. If you're
looking for cheap sources of 55 gallon drums check with your local
coke/pepsi bottler. They often sell these food-grade drums for dirt cheap.
Then it's just a matter of rigging up the necessary bits of tubing. Just
make sure you don't leave it directly open as that'd encourage mosquito
growth. If you have the space for it then set one up. Certainly can't hurt
to collect free water instead of paying the municipality or running your
well pump.