Also:
http://www.uline.com/BL_8250/?pricode=WM219
http://new.custom-magnets.com/product_info.php?cPath=2_47&products_id=562
http://www.magnet4less.com/index.php?cPath=4_70
There are 600 million magnetic sheeting companies. Anyway, it's not the cheapest option in the world, but I've found a lot of variety in price online.