Hello Steve,
At one time, Ford did offer a gasket for that lamp socket. But, to repeat a familiar phrase, They are no longer available from Ford, and I'm not aware of anyone offering a repo gasket.
Unless someone here has a spare socket with a good gasket, I would look at a repo socket and borrow the gasket from it. You could look at some of your favorite parts sites, but with today's shipping costs, the price escalates quickly.
Since just about everyone has a NAPA store, you can try them. The Ford part # for the socket is D1FZ-13234-A, which crosses to Napa part # ECH LS-6490. This part is from Napa's Echlin product line. While this socket could be slightly higher than some other sites, with no shipping involved, that helps keep the price in check.
If you have a favorite part store besides NAPA or an online source, they should be able to cross the Ford or Napa part number.