Here in Florida we get 12' X 2" X 6" fir stringers at Thomas Lumber for $95.00 give or take a buck. These are hand picked, high quality boards. We don't use fir plywood, but if you check a high end lumber supplier in your area and they should be able to help you get what you need. We use mainly ashe for toeboards and transoms and all our core is balsa. Hope this helps. Feel free to call if we can help you further.
Mike