Check with your favorite Mustang parts dealer and ask for a repop made by Daniel Carpenter. They make two versions now, one is a slightly cheaper one made in India - they had a bad run of these, so stay away from them. Get their original reproduction gasket they have made for years.
It is better than the original Ford. The quailty and fit is the same, but the Ford gasket had a small rib cast into it that makes it VERY hard to re-install the metal mouldings around the window. The Carpenter version doesn't have the rib and the mouldings go on much easier.