Our garage in London is currently repairing a 450 6.9 but we are having a hard time finding parts for our car, Mercedes main dealers don't sell much for this car anymore. I was wondering if anyone knew where I could get some rocker cover gaskets and a water pump for a 450 6.9.
Don't try at just any dealership. They'll fob you off, checking UK stock only.
Find a Classic Partner dealership, I think Mercedes-Benz of Poole are still one. They'll go the extra mile of checking German stock as well.
Failing that, check here: https://www.online-teile.com/mercedes-ersatzteile/index.php?language=en
I've bought the vast majority of parts for my own resto through them.
Failing that, eBay and eBay Kleinanzeigen are your next stop. There are people in the US who can refurbish the WUR, fuel distributor and hydro parts to factory condition and spec.
Except for the mechanical bits and hydro, most everything is interchangeable with the other models.