Unfortunately, they (dayseal) seems to be the only ones with replacement gaskets and seals for our vintage Sumbeams.

I would strongly recommend getting both when you replace the gasket, as the seal inside (accessible by twisting the 'cone' underneath the top half off) will most likely not be very reliable, if not failed already. Just be sure to tighten everything up VERY WELL or you will have a (literal) hot mess! TRUST ME, I KNOW. If you break down and get the gasket, a few dollars more for the secondary seal isn't really so bad- just consider it 'insurance' for proper operation.
Good luck!