Is it a manufactured gasket with a special shape or just the traditional square rubber gasket used in most older portlights? Perko still supplies gaskets for their long discontinued portlights: PERKO Inc. - Ventilators - Portlight Weatherstripping
If it's just the old style square rubber gasket, just google rubber and find a company near you. We used to buy it from Reller Rubber in Miami. If we needed 1/4" square, they would take a sheet of 1/4" thick rubber and slit off 1/4" wide pieces. As I recall the sheet was ten feet long so that is how long the gasket was. Same thing for 5/16" and 3/8".
Bomar uses round foam gasket. I think I've still got some of that in my shop. If not it's pretty easy to get. 3M Weatherstrip Adhesive is the correct glue to use.
If it's a special shape, you need to contact the manufacturer to see how to get them.