Bluezooaquatics has a nice guarentee but I rarely see them instock (just recently there were some)
Thatpetplace I had rough experience with and offer no guarentee but they often have them in as well.
You'd be surprised how hard it is to obtain a peacock, the trick is to stay alert for instock alerts on these places.
Liveaquaria does sell them by the name "Clown mantis shrimp" however they usually never are instock.. and infact they usually are never peacocks either. Diver's den section on the other hand post photos of the exact specimen, what you see is what you get basicly so that's handy, they have email updates for new daily diver's den specimens (I get those myself).