A list of creative reuse shops popped up in the comments from yesterday, I had heard of the ones in San Francisco and the East Bay, but turns out there are even more:
East Bay Depot in Berkeley. (A new site is in the works.)
Scrap Action in Portland.
SCRAP (Scroungers’ Center for Re-Usable Art Parts) In San Francisco. Not clear if they sell materials as well.
A whole big list of places all over the US.
Thanks to everyone who gave links!