I have a Gateway 500x desktop (waits for laughter to die down), and the power supply sounds like a small airplane. I bought a new/cheap one from Newegg, but it doesnt fit. The supply that came with the computer has the fan in the back, and one on the bottom side aimed at the CPU. It slides into place, and then a plastic bracket snaps up behind it and holds it in place. The new one that I bought is too big, and the screw holes dont line up.
most psu's are a set of standard sizes, http://www.pcguide.com/ref/power/sup/form_ATX.htm will help you find your current size, and from there you can find out if you can buy one. There are other sizes available on the open market you just need to know what you are looking for.