Did you do a quick product search here?
Penguin has one that meets exactly what you need here. Although, presently, 'out of stock', the resupply date is May 19th
http://www.penguinmagic.com/p/S3647
Or, just do a general search for 'mats' and you'll get a list of the many mats sold here.
Although, the sizes my vary slightly, you may want to check them out.
Or, search the net...there are many variation available.
Or, (finally), you could make your own.
Best wishes...