I don't think it's a woven product like PCB substrate. It's a rough random fibre fibreglass panel used in places like shower stalls, also known as FRP (glass fibre reinforced plastic).
Look at the surface material of the top right photo on this page:
http://www.bmsizone.com/products/FRP.html (ignore the honeycomb reinforced backing - it is available without this in plain sheet form).
EDIT: Bah! I spent too long looking for that link!