finding the easy to install vessel sink drain stopper
Searching for rubber drain stopper or rubber drain plug gets plenty of results. For example, Amazon - Danco has a range of sizes from 7/8" to 1-3/4". I can't vouch for this specific brand, but I have something similar in my tub. The basic idea is rubber will compress a little and if it is sloped a little instead of a cylinder, it can adjust to fit. Measure the drain and pick a stopper that is, ideally, a little larger than the drain at the top and a little narrower than the drain at the bottom.