Unless you live at the uttermost ends of the earth (Antarctica), there is a Freecycle.org list somewhere in your area.
When I returned to gardening in 2008 after a car-accident-related hiatus, I had just lost the last of three part-time jobs, so DH was holding the house together with his salary. Period. There was no money for "extras." Even the fact that, in California, seeds and starts for edible plants aren't charged sales tax was a factor.
So I read my local Freecycle lists very carefully. One day I hit pay dirt! A local business was re-arranging how he did his work, and was giving away a couple of dozen cinder blocks. I went and picked them up. I got them home and arranged them in a hollow rectangle, holes up, for Bed #1. Yep: just sitting *on* the ground, vs. in it. It was 4'x8'x7", the height of a cinder block.
Silly me: I believed Mel B about the depth. Bullfeathers... The plants that first season in Bed #1 did poorly, even though I gave them what Mel said they needed, albeit against what *I* thought they needed--MORE ROOT ZONE. So I started haunting Freecycle again for cinder blocks to run a second course of blocks for better depth. Even though my plants had access to native soil and I had created drainage channels with the pitchfork (I don't have a broadfork), they definitely needed more of the good soil that was in the bed.
Beds #2, #3, and #4 were made from Freecycled shelving of various lengths and widths. One of the beds was from 2x10s; the other two were from 2x12s. I didn't get all the wood at once, but accumulated it in two or three "hits." DH had some of the necessary hardware available here at the house, but eventually needed to purchase some angle brackets. (We didn't do the corner posts bit.) These are all 4'x4', and the first year we stacked them up into a potato "tower." Now I'm spoiled by the extra square footage and will need to make more frames if I want to use that potato-growing method again.
Finally, by September, I was ready for Bed #5. I wedged it in among my roses. It's 2.5'x12' long, 12" tall. This one was made from new wood b/c by then we were able to purchase the long lengths, but the short ends came from Freecycled wood.
DH *wanted* to buy new wood. I was perfectly willing to wait for Freecycle, but he had a Makita burning a hole in his pocket
and who was I to deny him that opportunity?
You're probably wondering about the cost. Well, it was very small. Beds #1 through #5: hardware, lumber (two boards for Bed #5), 1 bag of compressed peat (I was uneducated about peat at the time; won't buy it again), 1 bag of vermiculite (until the day I got some on Freecycle!) = approx. $54 total.
Just do a zip-code search on Freecycle.org and hang out for a while before putting your "WANTED" announcement up. Who knows? Someone may be putting up an "OFFER" announcement for exactly what you need!
Sunset Zone 17, USDA Zone 9