The paper mills make cardstock in large sheets (maybe 22 x 28, or even larger), and then cut it down to sell... I don't know that you'll find anything larger than 12x12 to buy as a consumer. If you have a paper dealer near you (look in the phone book, or online), you may be able to purchase some of the larger paper cut to whatever you'd like (though you'd probably have to buy a ream or more... not just a couple of sheets).