Steve, the first thing to do is identify the particular model of power supply required by your PC.
Once you have determined the correct model, look online for a replacement power supply. (Maybe ebay or amazon) There are many websites that specialize in computer parts and components and you can likely find a suitable power supply at a fair price.
You could also take your PC to a professional repair shop as an additional option. A technician can diagnose the issue and aid you in getting the right replacement power supply if necessary. While this may cost more than buying a replacement online, it can save time and guarantee that your PC is repaired correctly.