We use a combination of pallets and round wood. The bigger wood gets split. Another good source of wood is from anyone that operates a sawmill, especially the portable type. The bark slabs create some smoke, but usually have a good amount of wood on them. We can usually get good, dry red pine or aspen slabs free for the hauling.
The all-time best fuel came from a cheese packaging plant that one of my brothers worked for. Cheese factories used paraffin wax coated sheets of corrugated cardboard to separate and protect large blocks of cheese. A couple of these "liners" in the firebox would send a rocket flame out the top of the chimneys... spectacular at night. I believe that they started recycling the liners, so we haven't seen any for many years.
Scrap wood may be getting harder to find around here as wood pellet manufacturers are in the market for sawdust and wood pieces to use for their products.
Russell Berg
Mount View Sugar Bush - Since 1989
2004 5.5' x 18' Custom-built Stainless Evaporator w/ 7' Piggyback + Preheater
4,000 - 5,000 taps vacuum/gravity/bags
24' x 32' Sugar Shack