I had the same problem. I was able to find a 120V to 24VDC transformer at my local Home Depot. It is shaped in such a way that it fits in the box that my original thermostat was connected to. It was more than a few dollars, but worth it with what I've saved with the programmable thermostat. Unfortunately, I do not recall the brand name, but it was located in the same aisle as the thermostats. It was very simple to install - I believe 2 terminal screws for the 120V wires, and two small guage wires to connect to the thermostat. Good luck, and don't tell any 'thin-blooded' people in your house how to override the settings!! It always seems to be 70 degrees in my house!