I also like the Alvin mats. They are reversible, which gets twice the life out of them. They go up to 4 x 8' -- 48 x 96".
http://www.cutting-mats.net/self-hea...een-black.html
I use both a 24 x 36" and 18 x 24" and after two years of intensive cutting, some with a dull blade, I see no wear on them at all. I purchased mine from Amazon.