Question

Where to get Amulet Coin in B/W?

Answer 1

In Pokemon Black & White, you get the Amulet Coin by defeating 3 dancers in Castelia City. They are located in:

1. The dancer in Central Plaza near a water fountain
2. A dancer down the very left-most alleyway from the water fountain
3. Follow the alleyway down and onto a pier, and you'll find the 3rd dancer here

Now just return to the Castelia Central Plaza water fountain and you'll get the Amulet Coin from the dancer NPC.

Source: This page has step-by-step screenshots that show you exactly where to go and who to speak with.

Answer 2

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.

Answer 3

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