That is actually the Thermostat Housing.  I have the gaskets for $1, but if
you are going to work on it I would recommend replacing the whole housing as
they are made of plastic and get brittle with age.  The lower part is $24 and
the upper is $12.  Why not put in a new thermostat while are at it for
another $14 (stock).  I have all of these items in stock and ready to ship.
Just give me a call or order through the web store.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242