[bennelson]

Any place you can get free wood from is great.

Any place you can get CLEAN DRY wood is even better! (Cabinet shop, pallet-making place, etc.)

Pine can cause chimney fires, but not because it burns so hot, rather, it's the pitch or resin that's in the wood. That burns all hot and sticky.

If the chimney already has a build-up of creosote in it, an extra hot fire can ignite that. In general, fairly hot fires burn very clean. "Cooler" fires often don't completely burn all the volatiles, which can stick in the chimney and later catch fire.

Pine boards or 2x4s would be kiln-dried and should be fine for occasional burning.