You can pickup a new power supply at Fry’s, or any computer show. Any local mom & pop computer repair shop should be able to sell you one.
Before you buy a power supply though, you need to find out if the computer uses an AT or an ATX power supply. AT power supplies are extinct. If you have to have an AT power supply because of the age of your machine, then I would recommend raiding the thrift stores and finding an old computer that you can canabalize for parts.