Besides the usual hobby shops etc mentioned, you can pick them up at craft shops, and office supply stores, like AC Moore, and Staples. The larger the mat, the more it will cost. And yes, most aren't solvent resistant, which means liquid cement, and lacquer thinner will remove the grids, and dots. Just ask for "self-healing" cutting mats. They also come in different colors, but most stores usually carry just one brand, so you get what ever color they carry....
I have a grey one, and a green one.
Character is what you do when no one is looking.
Few things are harder to put up with than a good example.- Mark Twain
Make yourself an honest man, and then you may be sure that there is one less scoundrel in the world.--Thomas Carlyle