If you want to buy a 4x4 or 4x8 sheet try
http://www.professionalplastics.com/MICARTA. They sell in bulk and you need to check if you have a Regal Plastics dealer near you they also sometimes have sheets. I bought a sheet of black canvas micarta 15 years ago and am still cutting handles out of it.
Also try
http://www.angus-campbell.com/, I buy my thin (1/16th inch) micarta here for spacer material on hidden tang knives.