I am starting the process of restoring my 1955 Jaguar XK140MC. I have a high end mechanic that predominantly handles Ferraris and will be taking the lead in the restoration. I am looking for parts we want to use.

Does anyone know of a source for sodium filled exhaust valves

 

You could check aviation sources as sodium filled exhaust valves were commonly used in that application.
Having said that I wonder why you would want them in an XK140 as it would not be any great advantage and considerably more expensive.