If you are talking about the big rubber piece that goes around the perimeter of the windshield and holds it in, it is the same part for TR4-6 as well as early Spitfires. You can order it from Moss, Victoria British, the Roadster Factory etc. I have one that a snagged a couple places on a failed windshield install on the inside perimeter, should be functional but not cosmetically perfect, let me know if interested.
Greg