Are they rare or common?
It might depend on where you are. Here in the Central U.S., I see them from time to time. Every dealer in the UK would know what they were. Not every U.S. dealer will know. I bet if you went to any medium-size bourse, and showed the dealers a picture on your phone, at least one would say, "oh, I have a few of those...now where did I put them?"
Certainly online they're easy to find. You can search for test stamps, testing stamps, dummy stamps, "poached egg" and others.
You won't be surprised to hear that there is a group (at least one, maybe more) that studies dummy stamps.