As a repairman, I run into this issue every time someone wants to change from ceiling controls to a wall thermostat.
You have to run a 12VDC wire from somewhere.
These options come to mind.
1. Run a 12V wire from a source near the new thermostat location. You might be able to use a light source. Just make sure you use 12V that comes to the light switch, not the 12V leaving the light switch to the light.
2. Run 12V from the furnace to the thermostat. Tap into the red furnace wire.
3. Run a new 12V line from the fuse panel.
4. I have added a 120VAC Duplex box (THINK WALL OUTLET IN HOUSE) in the ceiling assembly area. Use the 120VAC coming to the new Control box. Just splice it in series prior to the control box. You can now buy an 120VAC to 12DC adapter (Radio Shack) Plug the adapter into the Duplex box and then you have a new 12VDC source. Since you have to run a bunch of wires from the Control Box down to the thermostat, you can run one more wire for the 12V to the thermostat.
Hope this helps.
