Each 8″ board should be around $15 and will give you 8 pieces of breaking material. (When I went, the final price per board was cheaper from the 8′ boards than the 10′ or 12′.)
Our students have gone to both Home Depot and Lowes and had the boards cut for free. They did it for me as well, but said they reserve the right to charge a fee per cut — in the range of .25 cents/cut. Cut against the grain for the 12″ squares.
When you chose your board
Make sure it is free from cracks and large knots. Especially take care that there are no knots right in the center where you will be breaking. (Small knots on the sides are fine.)
Choose wood with a wide grain that arcs. Students will punch into the arc or “under the rainbow.”
That is, unless, the board itself bows. In this case, you will punch under the bow (direction the wood arcs) rather than consider the direction the grain moves.
Ideally, and especially for smaller students, you want both the wood and the grain to arc in the same rainbow direction. For children, see the breaking material for kids post.
You don’t know who will be holding the board when you break it and you do not want anything to disrupt your chances of a good break–especially something like the board holder was readjusting their hands.