building a jet machine isnt as hard as it might seem. Most materials can be found at the local farmers co-op and plumbing suppliers. The expensive parts are the tank and the engine/pump combo. Most of the little 5hp trash pumps dont work that well so you need to find something with more hp. I almost burnt up my 5hp honda pump just trying to pump a slurry out of one machine that had broken down and that slurry was already mixed. Used machines are for sale all over the internet, jet and mechanical ones of all different sizes. By the time you figure your cost in parts and materials you can probably by a used one just as cheap. Even if you build one you still have to buy hose and nozzles to make it work and these should come with a used machine.
Refilling out of the customers garden hose is one source of water. I never liked that approach because most outside spicketts wont supply enough water to fill as fast as i can spray a load out. A nurse tank helps because you can just stick the hose in it and let it run while you are empting your hydroseeder. I usually fill from ponds and streams, I have a nurse tank mounted on a seperate truck. I just send the truck after a load of water while I am seeding. The nurse tank is usually back by the time I empty the seeder. I have also used my larger hydroseeder as a nurse tank for the smaller machine. It will hold enough water for two tank loads in the smaller machine. I only do this when I am seeding areas i am afraid to take the bigger machine into. I have used the smaller seeder and nurse tank to feed the larger machine to, but requires an extra driver so that doesnt happen much. I always go to a job with the hydroseeder full and the nurse tank as well, if the job requires more than one load. If a creek or stream is close, I wont use a nurse tank, I just unhook the hoses and take the machine to the water. I can adverage 2-600gal loads an hour this way.