Where can i get or what kind of power supply do i need for my computer?

i have a gateway SX2803-25e desktop computer. its one of those slimed down ones. it rececently died, not sure how, but my guess is the power supply needs to be replaced. after pressing the power on, little things like the keyboard, mouse, case light and cd tray work, but the light for the power on is out and the monitor doesnt register that the computer is on. The computer doesnt even make that single beep when turned on. so im guessing the power supply is bad....or could there be another option? if it is the power supply, where might i be able to find a new one for a slimmed down case?