My Miniature Speed Graphic came with one lens board, and I don't see this size on eBay (there's tons of them for 4x5s).

So where do people get them for other lenses, or do you make them? It doesn't look particularly hard, but my present one is made of 1/4" black Bakelite, and I don't know what modern plastic is equivalent to that. You could also make them out of wood, but hard plastic would probably be more stable.

If you want a plastic lensboard I would recommend Delrin, ABS or nylon. I believe you said in another post that you have machining facilities, and Delrin or ABS machine quite nicely. And they are both black in colour. But I make my lensboards from 1/8" basswood or 1/8" birch ply (both available at hobby shops or at craft stores such as Michael's), laminating the two layers crossways to each other to avoid warping and boring the hole out on the lathe.

1/4 inch thick black Acrylic is better than ABS for lens boards.
I use Baltic Birch plywood 1/4 inch thick. It's dimensionally stable.

Good point. The birch ply that I use is indeed the Baltic variety - it's what you find in Michael's. It has much thinner plies than ordinary plywood, which is what makes it so stable.