They are apparently called Aeolid nudibranchs. Aeolidiopsis is the genus of zoanthid eating nudibranchs that a quick search pulled up - one of the articles indicates that they may have either a strong preference for or a need for specific species of Zoanthids to eat (so one species of nudibranch may eat the zoas you want eaten, but others may not, there’s not a whole of info to go on here).
I don’t know if anyone sells them actively, but you might be able to find them at an lfs or in another reefers tank, or you might be able to special order some from a company that can go collect them (this would probably be very expensive, and I’m not sure of the legalities involved with such collections.)