They are great to have, mainly because they defend the Acropora from pests. IME they usually come in with Maricultured pieces. I have a few but they are very small so I rarely see them. I think LiveAquaria sells them seperately, but I'd be willing to bet you can find an Acro locally with one on it. Just gotta look real hard. Be careful not to buy a fuzzy crab, those eat the coral. HTH