Ok I use my silpats for lining my sheet pans when baking only and have never used them to roll out fondant but I don't see why you couldn't.
I just purchased a new roulpat fondant mat (because the one that comes with the sale of the biz is falling apart due to being very very old) that one is 24.5 x 16.5 and I got it at http://www.chefsresource.com/silpat-roulpat-liner.html
I reallized I could have probally gotten it cheaper else where but I just wanted to buy it and be done with it.
I did find a larger mat made by ateco that is 24 x 36 that I may get just to have a larger one on hand. That one can be found here http://www.cakesbysam.com/store/cart.php?target=product&product_id=21647&substring=ateco
I know from first hand that the roulpats are very long lasting, but do not have much experience with ateco. HTH