John,
If you want to have something that will be an exact replacement, I would try to cotact Modulus to see if they could help with the source. If they can't help you, you could try the solution that worked for my six-string build:
I used a Graph tech slab-
http://www.graphtech.com/products.html?SubCategoryID=57
Check the necessary thickness of the nut...since the slabs are only 3/16" thick, I laminated some contrasting wood veneers to the headstock side of the nut with CA glue, trimmed it, slotted it and it looks great.
