This is a graphic arts product called "Color-Aid", comes in just about every color, shade, tint you could want. The surface is silk-screened on and is very durable, the matte finish prevents unwanted glare (the black paper seems to soak up light, better than black velvet).
The sheets are 18x24 inches, which are cut up and mounted (spray glue) on pieces of acrylic water color paper (although not water proof, illustration board will work with the white side out), 7x8" seems the most often used, a range of sizes is handy in the field and the white side is good for bounce lighting.
The Gray #2 (Gray 4.5) is the one I prefer (for now), is close to the Kodak 18% card without the "sparkle bits" seen in magnification. The other levels of gray could be used to adjust subject to background ratios for the camera's sensors.
If you find a source, stock up on the gray you like, supplies seem to be getting scarce.

I will definitely look into that.