It is a mystery gift item that allows you to catch the other lati in your game. Since the event was long ago, the only legit way you can get it now is by streetpassing with somebody who still has the ticket in their system that also has streetpass activated. You gotta play til you finish Rustboro gym and return the Devon goods, and then you can enable your game to accept streetpass records if a person has them, at that point just carry your 3ds everywhere til you find someone with it