No such animal anymore amnit: Ford stopped making the gaskets in the late 80s/very early 90s (My new gasket set is circa 1994 and it's repop) crapola MCA discontinued the MCA Approved Parts program due to liabilty issues, so there's no such thing as an approved part anymore, in any way.
Note that the repop gaskets will work fine if installed with care, and lots of goop in the correct places.
Do you have an original or replacement windshield? The gaskets that are avaiable are actually fine when used with original windshields but new windshields are much thinner and this leads to sealing problems. You just have to use a lot of goo to get the job done.
If you are lucky enough to find a good Ford parts guy, there might be a few originals left. There is a dealership about 30 miles from here that has a guy that gets obsolete Ford parts that all other dealers say don't exist.
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.