Go to a nice lumber store and get some hardwood. In the Bay Area I go to PALS
and buy certified hardwood. Certified means that the wood was grown and harvested in a sustainable way.
The wood will be measured in board feet. Roughly, a board foot is a measure of volume equal to 144 cubic inches; however, as with any trade specific measurement, it's not quite that simple. Rounding errors need to be dealt with properly