You're looking for the Mirage Forest north of Route 111.
You'll have to wait and keep checking every day to see if the Mirage Forest appeared.
>A Mirage Forest is a location in Hoenn located on an island in Pokémon Omega Ruby and Alpha Sapphire. It is only accessible by soaring on Latias or Latios. A Mirage Forest can appear daily, or it may not appear, being replaced with a Mirage Island, Mirage Cave, or Mirage Mountain. More Mirage spots can appear via StreetPass.
Perhaps try Streetpass, if you're totally out of luck.