Thanks to this thread, I already found a leftover from somewhere.

However, the tabs are so small, that using the "vent" once will do it in. Apparently they were made never to be removed, and if so, new ones would be used to replace them.

They were a poor design, as they are uniform (from piece to piece) but the holes in the cast wheel are not. Therefore, they don't fit just right. But . . . they served their purpose and were probably pretty cheap to replace so BMW went with them. Now . . . 36 years later, the vents aren't so plentiful, dog-gone near impossible to find! I guarantee, mine won't fall out again!

I am intrigued by the idea of getting some made by the 3-D printer. While I don't have any available to detail-measure, I could probably get a pretty reliable sketch together to see if more could be made.