http://geistwerks.us/main/index.php/201 ... -22-26/2x2

Its probably something like a 48" x 22". I've seen a similar one on ebay but in white but this one has the mill finish. In a few of the photos on the websites gallery you can see the interior of the door too. I suppose I could ask the builder where he gets them but since he's in business to make and sell teardrops I thought it might be rude to ask for sources of materials.
I'll most likely just plan for building my own door but a prefab door is quick and easy to install, although usually pricey. I'm planning a new build that will sleep 2 adults and 2 small children so its going to be larger than my previous builds.