The town nearest to Twinleaf. This is where your second rival lives, which is either Dawn if you chose to be male or Lucas if you chose female. Its key features include the beach in the south and Rowan's Lab.
Locations of Swarm Pokémon can be discovered here. Go to Dawn/Lucas's house and talk to his/her sister and she will tell you that (let's use Beldum as an example) Beldum has been spotted at Route 228. So to that route and use the PokéRadar to find Beldum