You might want to look at Segal's too. There are a number of classic car places that do it, as Terry says. Segal's have some plain if you don't want the embossed effect.
Ratchford's (the book-binding company) also sell grey millboard. I have only bought their leathercloth, but they distinguish between the millboard and 'greyboard' which I think is the solid cardboard used on the backs of pads of paper. Perhaps the millboard is extra solid.