In castelia city, in the north fountain, there is a dancer. Talk to him, then find his buddies, one at one of the piers the other in the narrow road, and then they will give you amulet coin.
You get it in castelia city after you unite the three dancers(Edmond, Mickey, and Raymond) at the water fountain. Source: http://bulbapedia.bulbagarden.net/wiki/Amulet_Coin#Amulet_Coin