Can anyone tell me where to locate the slave cylinder for a 1978 Mercedes Benz 450SL?

You have a 450SL with manual transmission?

The clutch slave cylinder is usually ahead of the bell housing on the opposite side of the starter. Look for the clutch fork sticking out of the bell housing. The slave cylinder is the hydraulic ram that actuates the clutch fork.

There could be other kinds of slave cylinders I don't know about.

Sixto