Unless you have some spare parts laying around you can just about buy one for what it will cost you to build one. If you have parts that you can use you can buy seperate parts and build one or rebuilt an old one. If your planning on spraying more than 10 acres I'd get a tractor mounted one with at least a 200 gallon tank & 20 foot boom min.
I have everythig needed to build one on a skid and either strap it to an existing trailer of mine, or else attach a 3-point hitch to it. I am still considering buying one since I don't really have the time to mess with building one right now.
RC, if do a search in the tractors/equipment forum (sprayer), you will find one I built. I think it ran me around $250 to build. Most of that was the pump, nozzles, pto adapter and such.