You can find it during a swarm
I tried everything! I knew its at route 9 though. But you need to be there on a specific day like the guy said. I'm trying to do every day in the month. I got nothing yet but sometime I will.
Go to the Poke-Gear and then go to the radio. Go to the radio station on the top right (The Pokemon Show) and it will be announced on the radio if it's the day Mawile shows up.