I would also have to strongly suggest against smoking anything with cedar.

For the BBQ the planks are soaked in water for about 2 hours and you are supposted to put out any flames with a water bottle. In this process there is very little smoke delivered from the cedar. It mostly keeps the fish from being charred and transfers the heat well because cedar is very porous. I don't believe there is any reason why you can't cut and use any piece of cedar from the woods. As long as it is very well seasoned. The boards they sell in the store are nothing special. But I could be wrong.