Sunbeam Coffee Gaskets:
Alas Sunbeam long since ceased production of their vac pots, IIRC the 1960's model "C-50" was the last of the series. You may be able to find NOS gaskets lying around, but they are pretty rare. There was talk over on the Interent group rec.coffee about clubbing together to have a mold then more gaskets made, but don't know what became of that project.
Depending upon the state of your gasket it may be possible to soften it up by soaking in ammonia or glycerin (Goggle Internet archives for exact directions), but as one has never tried either method, cannot vouch for sucess. Most persons simply purchase another similar Sunbeam vac pot, or at least the upper chamber to get a good gasket. Sold off a non-working Sunbeam vac pot to someone who only wanted the gasket. As with most other vintage appliances, it is not uncommon for vintage vac pot users/collectors to have several pots on hand, mainly as a ready store of parts and accessories.
How To Use A Sunbeam Vac Pot:
Models vary slighly, but basically directions are the same. One pours cold water into the lower chamber, insert the upper chamber then "clamps" down to secure. Add correct amount of coffee to upper chamber (one should have inserted the strainer beforehand), put lid on upper chamber,turn on unit and wait for the unit to do it's thing.
Can dig out my C-50 directions for a blow by blow description, but do not have a good scanner to make a copy.
Was lucky my Sunbeam C-50 came with lots of extra "goodies", such as extra strainers and filter cloths, so was set as far as that goes. Am still on the hunt for another MIB unit to store away for use if and when my current unit dies.
If you need other assistance, please ask away.
L.