Can find in a hidden chest.. or buy from lion trading, convert gold into gems, buy buy direct 5 pieces (more cheap)..
It is possible to receive a Black Lion Key from opening a Black Lion Chest. So meta.
I found one black lion chest during yesterday's stess test, dropped from who knows what monster. The thing is I'm unable to find the chest in gw2db, yet I've ovbiously found the key to open it (which, by the way, I couldn't find ingame...)