There are some 12x12 mats out there (other brands) -
http://www.amazon.com/Westcott-Self-.../dp/B001D6KORY
Isn't this one adorable (13x13)?
http://www.joann.com/self-healing-13...k-/xprd646736/
However, if you have an old mat I would be tempted to cut it up and try it out instead of buying a new top.