"One variety of the mysterious Mega Stones. Have Baariette hold it,and this stone will enable it to Mega Evolve." Baariettite can be obtained from The Labyrinth in a small room (Pictured below) that can be accessed by surfing up and then left from the Route 13 entrance.
Baariettite can also be obtained if you have a Baariette in your party after defeating gym leader Vaeryn and you select that Baariette after being asked what Pokémon you would like to be able to Mega Evolve.