You guys with the Sen-Dure coolers, which is many of us, Lehman, Cummins, Westerbeke- The gaskets are generic and available in any good marine supply house.
But don't forget the little rubber donut for the central bolt. Most covers are stamped brass and somewhat concave. Don't overtighten as it can crack the cap, fortunately caps available too.
I do as SD and smear a thin coat of Permatex on the OD of the cooler right before installing cap and gaskets. Keeps it from weeping, which is what causes that corrosion product. Really harmless if it does show up.