There are a couple of methods depending on the force you need....
you can tank a metal weed sprayer and add a shrader valve which will save you the hassle of pumping.
you can also do something like the link above which is pretty cool.
However I really recommend you get a good compressor, I have used Extreme Outback on a few rigs and beat the snot out of them, mounted under my rig on the frame rail and the 100% duty cycle means the just work. Family owned company that will take care of you.