Miker, if you are missing the asbestos fibre cement gasket between striker plate and jamb, you can cut out a flat rubber pad and it will work just as good. but you'd need to get an atomic screwdriver to tighten/loosen the screws. If your car has the hourglass head screws, the special hourglass screwdriver will suffice, but if they are the phillips-head screws, get the biggest phillips screwdriver you can find.
I think you can still buy new composite gaskets from some of the repro parts suppliers, but I don't know if their anti-slip ability is any good.