An air compressor has three main parts, a pump, a motor, and a tank. To get these you can go to you local scrap yard, they usually have a few scrapped...
This is how to make a cheap aircompressor from parts from the junk yard, for running airtools, filling tires and pneumatic spudguns.
Step 1: Safety
This isn't quite a set of instructions but more like a guide line. Because I made this from scrap, none of the parts are standard. Also take safety into account when working with compressed air, old tanks and rust out and explode with terrible results unless you know what your doing dont make one by one. So if you make one and it bursts and blows off you leg dont come crying to me.