The windshield gasket is available from the club. Join KFOCI.
The rear window gasket is NOT available from any source. I replaced mine with new top and bottom gasket material from Steele rubber products, I re-used the "dog-leg" corners and the divider gaskets l had available from my parts car which I owned since the 1950's. (Stored inside since the 1960's).
I used a rejuvenating liquid called Leatherique that I found available on-line. It claimed to soften and rejuvinate leather, vinyl, & rubber. ( it did "partially" soften the old rubber) I also used it on other rubber parts like the rubber around the rear window vents which I didn't replace with new rubber.
Replacing the rear window proved to be very challenging as the gasket was in pieces. (We used 3M's
Weather striping adhesive to seal the joints) we installed the glass in thirds, installing the side (corner) pieces first then the center section.