well pelts run at maximum efficiency @ 85% there max rated voltage, or thereabouts so if you had a 12v pelt you woult want 10.2v @ however many amperes requierd, any power supply will do as long as its dc and meets your current requirements
Do a Google search for 24 volt PSU. Depending on the load, you pelt will probably like something between 18 and 22 volts. Most 24 volt switching PSUs have a trimmer that will allow you to dial it down to 22 volts. MJPA often has 24 volt PSUs. Ham radio suppliers sell 28 volt PSUs that will work once you adjust the voltage. MFJ and Astron are the main 28 volt ham PSU manufacturers.