There are many places where you can buy mat board, but the finest board can be found at Superior Archival Materials (http://www.superiorarchivalmats.com/ as someone already mentioned). See my article on "Advances on Archival Products," which first appeared in View Camera magazine. It describes why this board is better than other boards. Some surprising findings--that normal 100% acid-free all rag board, the kind you get from Light Impressions and other places is not truly archival, but the 100% acid-free all rag board that Superior has, because of the technology of making it, is truly archival. When I learned this stuff, I immediately switched over to the board that Superior carries. My article can be found on the Superior site, I believe. It can also be found at www.michaelandpaula.com under "Writings."
Michael A. Smith