Ask for materials, and look at what people are offering, on any FreeCycle lists near where you live. Go to www.freecycle.org and do a search on your town, county, regional name, zip code, or whatever makes sense to see what's out there.
There are very few rules on FreeCycle, but ironclad is that there are NO cash transactions, ever. Also: everything must be "family friendly"--no alcohol, risque materials, etc. Gardening is *certainly* family friendly!
I got my cinder blocks and much of the wood last year to make my raised beds/Square Foot Gardens. I have a mix of pine shelving and 2x12's now.
Sunset Zone 17, USDA Zone 9