I came across this really great resource for finding used/extra building materials for sale in your area. It's basically a Craigslist just for home improvement/building materials/appliances, and it's called DiggersList. Just type in your area/town and DiggersList will find anything for sale/free in a 50 mile radius. I really like the name, because as someone who loves to find used materials in the dumpster or in piles on the road, or even in piles on the internet, I think people like us really are "Diggers!" If you check out DiggersList and find something great to use in your tiny house, camper conversion, or other project, let me know in the comments!
(And on the flip side of acquiring great used/unused extra building materials for free or cheap, you can also post your own leftover flooring, old fridge, or salvaged lumber on the site so others can take it off your hands!)