If you havent found the answer it basically a block of laminated birch, approximatley 25-30 sheets individually dyed and glued together.
I believe the main supplier of this material is Rutlan Plywood company, the material is called stratabond. You can obtain it from them, but if you want a smaller quantity you can also get riser blanks from Bingham's (called coreflex superaction there). Grey coreflex superaction is my favorite material to use...