As Neil Jones had mentioned, you are not likely to be able to just replace the power supply fan, you potentially could injure yourself by doing so. However, that said, you pretty much would have to look at the size, voltage, and current of the existing fan and set out searching for a replacement. The problem is that virtually every power supply I have seen and taken apart, the fan wires on the power supply are soldered on to the board. You would have two options if you manage to find a fan from a local computer retailer etc, unsolder the old fan connections, and solder in the new, or clip the wires for the old fan and use some wire nuts to rig in the replacement fan. I would recommend replacing the entire power supply however, due to time to replace the fan versus swapping out the new power supply and to prevent the chance of bodily injury, $50 is not much.