You can't beat this for the thrill of the hunt. Habitat for Humanity's ReStore outlets are located across the country. Recently, we checked out one in Portland, Ore. We were amazed and delighted by what people give away: doors, paint, cabinets... a good portion of it new and unused, the result of measuring error.
Let's say you absolutely need just a bit of a synthetic paint, thinner, or sealant? Chances are your local ReStore has it in stock for not much money... and if you're lucky, they'll have just enough for your project. If you think about it, this eliminates waste twice, and you can always donate your leftovers.

New, unused windows are a bargain, priced simply by the square foot.

We spotted a couple gems, including a farm sink in near perfect condition and beautiful grey-blue polished porcelain tiles. But, as this sign says, you have to sort the wheat from the chaff.
Thanks to AT:Green Home Reader Kathryn for the tip!
This is such a neat idea, and I'm pleasantly surprised to find one in my neck of the woods! Thanks for the tipoff!
view Anne (in Reno)'s profile
This kind of store can also be an excellent place to score affordable historic molding and detailing, if you live in a region where there are lots of century-old houses or apartments that weren't high-end enough for their trim to be sold to "architectural antiques" shops. The Habitat store in South Minneapolis -- it may be gone now -- was HUGE for this.
view wende in the twin cities's profile
In Boston we have the Building Materials Resource Center, a non-profit that collects donated building materials for resale. http://www.bostonbmrc.org/bostonbmrc/index.html
If you or your family are considered low income, you can get a substantial discount. Also, non-profits can get a discount.
Bonus, they offer workshops on DIY home repair, green home projects, etc. for only $10 per class.
view cantabrigian1's profile