Mesprit will run off to a random route in Sinnoh. You can pinpoint its location on the PokéDex app that has the map of Sinnoh and the little blinking cat face the symbolizes roaming Pokémon. Do not fly to where it is, instead run back and forth on the border of two areas until it comes to you (I find Sandgem Town and Route 202 to be the best places).
