I don't believe there are gaskets available for the breather covers. Even searching through Mercedes-Benz sites, everyone reseals them with RTV silicon.
If you decide to reseal the breather covers yourself, my advice is to take your time and make sure that everything is very clean and dry.
VERY IMPORTANT: Do not use a wire brush on the breather covers. They are made of magnesium alloy. Once the coating is gone they will corrode over time.

There are videos available on youtube that show how to reseal only the breather covers. Search for Mercedes M112 breather covers.
Best of luck to you.