Serebii elaborated on Mesprit; it appears at the top of Celestial Tower.
It's Level 65. It's at the top of the Celestial Tower following a request to search for them by Professor Juniper.
The implication of this is that all three Lake Guardians are in B2/W2 (though Azelf has yet to be found/shown). Whether this also holds true for the Regi trio, we'll have to wait for word.