Yardsticks are used as the spars/struts in the interior of wings. When choosing yardsticks, try and find ones which are as straight as possible. Also pay attention to the direction of the grain of the wood. Ideally it should be along the short direction (up and down if the stick is on its edge).
Yardsticks can be found at home improvement/DIY stores.
- ACE Hardware
Some stores that apparently do not carry yardsticks include: