I think Black Perculas are a myth and don't really exist. I've seen some beautiful "Onyx" Percs that have a lot of black but never an all black one. Rod of Rod's Reef used to sell the Onyx. I'm not sure if he's still breeding his pair.
If you're referring to black ocellaris, which are commonly known as black percs, a number of online vendors carry them (off the top of my head, liveaquaria, themarinecenter, marinedepotlive). I believe they're being farmed by ORA. HTH