Check out your local Habitat for Humanity store. I'm a regular shopper there. Have bought some awesome windows, screens, and hardware. They have lots of doors available for around $7 - 15. You can also find some useable materials at your local dump, side walk trash day, craig's list. Lumber yard seconds. Happy shopping! If I were you, I'd get your door and window(s) before you start building. It's a lot easier to plan your framing when you have the r.o.'s already planned, than to try to do it after the fact. Also, if you have those components on hand, they will help with your design inspiration. An other thing you might want to check is : Automatic pop door using a car radio antannae and motor.