Watch glass are round, concave pieces of glass. They’re available in a variety of sizes. Mine were purchased from Grainger Canada.
In the photo for this entry you can see a 10cm watch glass functioning as a lid for a small beaker.
I get a lot of questions about these, but honestly, I don’t think they’re very useful for most formulators. I mostly use them as a convenient flat-ish surface for displaying ingredients in video b-roll. If you do not make videos and find yourself slowly rotating a small plate of shea butter/clay/etc. in front of a camera on a regular basis, don’t waste your money. Glass Prep Cups are much more useful as they’ll hold both liquids and solids. You can use tin foil or other flat, solid things as beaker lids.