I used to be able to get a lot of wood from the city.

They are always cutting trees and would get large trunks and branches they couldn't feed through a chipper or sell because all the mills were too afraid of nails in the wood.
Some of the tree trunks were just way to big around for a commercial mill too.
You might not even need to go pick it up yourself, but I had to go to a yard they used for dumping them in. I had to sign an insurance waiver to be allowed to cut it up and load it.

I found it was a bit hard on saw chains, but with a bit of caution it was not as bad as you might expect.

It might be worth calling whoever deals with parks or maintenance in your nearest city.