If you mean the front windshield rubber gasket/seal, they should be available from almost any Mustang supplier or possibly any local glass shop. Don't pay too much for a windshield. I just had a PPG put in SWMBO's '66 for about $180 including labor. I supplied the new rubber.
I have not found 1 aftermarket parts supplier that has the convert front window seal they all say it has been nla for years. All the local glass shops are telling me that I need to supply the gasket and they will install new glass for right at $200.00