Bought and installed a used sink, but the drain control lever cap was missing. (And the old one was, of course, different.)
The HW store does not sell the cap alone, so you're looking at a new $40 complete drain! Come on! If I had that kind o' cash to waste I wouldn't be installing a used sink!
The fit isn't 100% perfect on this part, but with the proper application of heat gun and violence it goes on well enough.
There is also a non-hole version if you want to remove the drain mechanism entirely.