It seems to be a little-known secret that scrap lumber will burn. It seems everybody wants decorative wood. Scrap lumber is so hard to get rid of that demolition contractors will not only give it away, they'll deliver it. If you're closer than the dump, it's cheaper to haul it to you, and no tipping fee.
I've also gotten truckloads of big (3'-4' )rounds from a contractor who was doing park work. They were too big for anybody else to bother splitting and he wasn't in the firewood business anyway.
Another delivery was several tractor-trailer loads from a power line clearing project (Wind farms are a big deal around here) The contractor was from over a hundred miles away, so only the best of it was worth hauling home, and he couldn't legally move ash out of the county. He had to dispose of it locally.
The catch to all of these deals is that you have to take the bad to get the good. To get the lumber, I had to take the naily, splintered boards. (But on the plus side, there's usually some usable lumber in the mix) To get the big tree trunks, I had to take the punky wood (after all, that's why these trees were being cut). To get the power line clearings, I had to take the brush. Fortunately, I have plenty of space to let the unusable stuff just sit and decompose (I did chip a lot of the brush), but if you're in the suburbs, this might not work for you. Still, the point is you have to be willing to take what nobody else wants. It's a lot harder if you want to cherry-pick. When somebody calls and offers to deliver something for free, I never refuse. If you say "no" too often, they might not call when they have something good.