If you mean Pseudocorynactis, they frequently come as hitchhikers on live rock. I have a couple small ones in each of my tanks. They are strong stingers so some people consider them pests but I think they are really cool looking. I've never seen them "for sale" anywhere.
They are non-photosynthetic so if you don't have an established tank with lots of microfauna you'll need to target feed them periodically.