In most urban areas, there are places where you can donate the scrap parts of your home. Habitat For Humanity has a service where they will come and take apart your building and resell the material. I think it was the Restore in Portland where we got all the stuff for the railing we built with the Omeys.
The most reliable way to find scrap building materials is just to go get it physically. You could search the dump. I don't know if you'd want to, but that's an option in some places. You can call construction companies and see if they have anything available. Other than that, keep an eye out for people who are tearing things down or any one of the usual websites: Freecycle, Craiglists, etc.
The Omeys used a number of different methods to obtain used building materials, but Mr. Omey searched on Craigslist extensively. He said, "It's amazing what you can find on Craigslist."
Carport on a Dime
A lot of the materials that Nelson used for his carport were in tough shape, and it wasn't free. There is nothing free. As in anything, it was a trade-off. You're going to have to hunt it down. You're going to have to go get and transport it back to your job site, and you're probably going to have to modify it some way. In this case, a lot of the materials needed to be repaired.
However, it is technically free, dollar amount-wise. Plus, it's recycling.
Don't miss the Carport on a Dime episode Steve writes about.