|I believe that's a desktop, right?|
First of all, do you think it's bad because your computer won't turn on? If so it's usually a problem with the computer and not the switch. If you open up the case and locate the wires going from that switch to its corresponding pins on the motherboard you can remove the connector and momentarily short those two motherboard pins with a metal tipped screwdriver. If it still doesn't start then the switch isn't the problem.
If the switch is broken most of the ones I've seen are fairly generic and are attached to a small bracket that screws onto the front metal chassis, under the front plastic cover of the computer. You shouldn't have a problem locating one online or at a electronic parts store or just pulling one out of a trashed computer.