I've been looking for a silpat that fits my 2/3 sheet pans for years. Well, I finally found one made by Winco (SBS-21PP) that is 14 7/16 x 20 1/2 and fits the pan perfectly. (It's purple, which is restaurant code for 'allergy free', but that's OK by me.) I'm looking forward to seeing how it affects bread, it should keep the bottom from getting overbaked.
My fall back option, and I'll probably order it anyway, was a 16 inch x 5 foot roll of silicone material with 1 inch grid marks on it, available on Amazon for around $20. It comes in red or blue. This is pure silicone and can be cut to shape to fit the pans you have, unlike the silpats which have fiberglass in them and cannot be cut.