Become a Full Member and get the official Linkage Tour and the official Starting Aid. This information alone will pay for your membership
Cardone rebuilds the ATE boosters for $115. Turn around time is about two weeks. https://www.rockauto.com/en/catalog/mercedes-benz,1967,250sl,2.5l+l6,1434935,brake+&+wheel+hub,power+brake+booster,1884
If anyone ships a booster be very careful to pack it so the plastic nose is well protected. They are not available anymore so if it gets broken your booster cannot be rebuilt.