I suggest to pick up a new one and have it just in case. My motto is always better to have and not required than require it and not have it. Personally when I installed my Moutnune CBE I reused the stock gasket because it only had 2000 miles on it and it was perfectly fine. It is fine and have no leaks now at 18,000 miles.
Any muffler shop should have one, worse case the Ford Dealership should have one.
GL