We collect leftover pieces of wood from all kinds of projects (storage TBD).
While the small pieces may only be good enough for burning eventually, the larger pieces have great potential to be incorporated into projects one more time! They are also useful as test pieces, e.g. for laser cutting.
Caution - recycled wood may contain nails, screws or other metal parts. Take care to remove them or cut/drill somewhere else if you can't!
Another way of getting very cheap or even free wood is using reclaimed wood.