I don't believe there are any places that sell panel ray parts.
Your best bet is to match something up that will work. Take a look at Grainger or another major supplier:
You may find that you're best off using a low-profile 750 mV control, thermopile generator, and a remote thermostat. The parts are available, you can find something that will fit, and it will work well. Manual controls that will work are, at best, hard to find.