keep an eye on the free section of your local craigslist
i know mine has all sorts of free wood on there
i picked up one or two old decks that people replaced, must be 2 years ago now
that decking is my "go to" wood
ive used it to fix a broken bridge in the back yard, make a new ramp for one of the sheds, build a set of stairs for the kids to get unto the trampoline, even a front deck for another one of the sheds...all of which cost me nothing but time and hardware
i wouldnt think twice about building shelves out of it
