It sounds like you are talking about a seal for the metal framed vent to set against when the vent is closed and not a seal that is in contact with the glass itself.
To get an even pressure compression seal, why not just get some weather stripping in roll form. It comes in many widths and thickness', just be sure to use the closed cell type. (Open cell would abosrb water like a sponge) You could glue the butt joint together where a splice occurs.
The original type of seal material may still be available, but chances are... you may need to disassemble the main frame to replace this stuff because it usually slides into a channel or slot.
Try looking through the online catalog at
to see if you can find the original gasket.