There are still iris clamps made for optical testing. What they lack to be useful is a way of mounting them on a lens board, and (I think, but I'm not sure) size.
Iris clamps, or iris universal lens mount, seem to be the most common "names". But many sellers seem to think they're big apertures, so a search won't find much.
I wouldn't say they are rare, but you will often have to buy a whole camera to get one. They seem to be fairly common on German plate cameras from the first few decades of the 20th century - that's how I got two of mine. The first one I bought to use on a different camera, but found the included camera too nice to butcher. Did that once more, and then managed to find two without camera attached, and a small one with a big box of miscellaneous parts and pieces...