I bought the big horse stall mats. They are extremely heavy, durable, and after 2 years, my dogs have not been able to chew or dig through them. The only downside is that they are BLACK, and thus, they do get hot in the summer, unless you paint them a neutral color. These have worked great! Easy to clean up, sterilize, etc. They run about $40-$80, depending on size. I get the 4x6's for about $50 each. You should be able to find them at a feed store, or have them special order some for you. Make sure you have some friends with you when you are placing them--when I say HEAVY I do mean it!!!
Hope this helps. :)