Mat board is thick board made from wood or cotton fiber pulp and is most often covered with a colored or decorative facing.
Most board is 1/16, but you can get white cotton in a double thick form making it 1/8.
You should be able to get the thin board from most locations, for the double thick you will have to go to a frame shop (Hobby Lobby, Michaels, and other hobby chains have frame shops in the store)
Both tend to crease if you bend them dry, but if you steam them they should bend reasonably, I'd be very careful about actually wetting the boards.
Mat board is easier to glue together then foamcore and is denser, therby more resistant to pressure. Most framers use an ATG (Adhesive Transfer Gun) to bind mat board layers together.
It would make a good core for fiberglassing or as a mold for making multiple copies.