Teak Lumber can be acquired by using a Saw on Teak Tree or Driftwood of Teak.
You can also smash Violins with a Hammer to get Teak Lumber.
This can be used in the following recipes:
From the ITEM menu, use a Saw (or better) on Teak Tree (5x Teak Tree = 5x Teak Lumber) or Driftwood of Teak (1x Drifwood of Teak = 1x Teak Lumber).
Use Hammer from the TOOL menu on Violin (3-8x Thread).