I just like using precut, untreated, cured wood from a nearby mill, or if I want to be really ghetto, I see if anyone has wood pallets, yank the nails, and use those. Pallets are untreated wood, so either will burn with a lot of energy compared to plain old firewood. The European pallets with the pressed sawdust spacers also burn well too.
One note: Ask first. Wal Mart does NOT allow people to take pallets (the manager I asked said they watch the cameras and actively prosecute it just like shoplifting), but other places likely don't care, but ask to be safe.