I read someone on here used an Alvin brand drafting mat. I bought one and I LOVE it! It is the best mat I have ever used. I have tried Fiskar's, June Tailor and Olfa and the Alvin one is far better. I bought it from Amazon. I believe the otehr lady who talked about it was able to buy it at a college book store. You could also try a drafting supply store.
It is so thick and it doesn't get the grooves like the other mats did. I love it!
Oh, they come in multiple sizes, too. Mine it 24" X 36".