Go to a place that sells stone and granite and have one cut to order. Or go to your tile place and buy marble tiles....they are ground to fit together very tightly, or get a piece of granite cut.
Linda C

Also, check this Google search on "marble pastry board." There are a bunch there, and maybe one will be the right size and price. They all seem to be much less than Williams Sonoma, which is usually the case.

Here is a link that might be useful: Google search

The stone and granite places usually have extra end pieces left over from their countertop orders. Try to find out if they just throw these out. If so, you could probably either get one for free or real cheap.

Thanks for the advice, I will check around to see if we have any local stone fabricators.