They are popular among those who know about them. Most stores will give you some for free if they have them.
Also, often they come as hitchhikers with frags, corals or live rocks.
Also good are the little limpets that often come as hitchhikers on live rock (the real ones from the ocean). Usually you find their offspring in the skimmer reaction chamber or at the end of the foam tube. They also like to live in the overflow box - everywhere where they find highly oxygenated water and strong flow.
In the wild you usually find them on the edge of the tidal zone where the waves break on the rocks.