Is it made of anything special? If its missing, where could I pick one up?
It's special in that it's a gasket but made to handle 1,300° without melting. Permatex RTV-type gasket makers give up the ghost at around 500° or 600°, so they won't do the job. I'd contact Bob at DieselSite or David at Diesel Manor and ask them. Maybe some sort of a gasket material available at auto parts stores you could use to make your own?