I made the door, and frame myself. The heart of the pop door is an "Add A Motor D-20" that I ordered from Sears Online (best price I could find).
Below is the link of my post with the materials and prices I used to make it. It's been working with my chickens for over a month now and I couldn't be happier with it's performance. http://www.backyardchickens.com/t/628008/automatic-pop-door