Hi Tony, these valves are expensive alright even more so if a 20v with sodium as has been mentioned. Had same problem myself trying to source from Audi AAN exhaust valves but NLA having only a few so decided to buy a better than original and sourced Inconell valves form this place.
https://www.s2forum.com/member/11134-fcp-engineering in Lithuania,i'm sure he will be able to supply MB valves
These are available in the States but very expensive and then there's the import duty.