Instructions
Defeat Flannery, the gym leader in Lavaridge Town, and exit the gym. An event will automatically take place.
Receive the "Go-Goggles" item from your rival during this event. These goggles allow you to enter the desert despite the sandstorm.
Travel to the desert area north of Maviulle City. This is the only location where you can encounter the Trapinch.
Battle the Trapinch and capture it with a Pokeball.
Place the Trapinch in your Pokemon team and use it in battle to gain experience. It will evolve into the Pokemon Vibrava at level 35, and then into the Pokemon Flygon at level 45.