The cruise control stalk of my 1980 240D was also broken off when I bought it. The one in the 240D I spotted at the junkyard was also broken. But I managed to spot one in the back seat area of a 300D that was almost completely gutted.
On my W123, the cruise stalk is mounted in front of (closer to the driver) the turn signal lever. There is a rubber flap that covers the switches and has a slit that looks something like this:
O-----o
The larger circle is for the turn signal lever, while the smaller one is for the cruise control stalk and is easily missed. That rubber flap pulls right off with very little effort. Hopefully, yours will be similar and this will be useful.