I believe that the amulet coin is located in Amity square (hearthome city) I think you have to go in on the right corner, make sure to bring a pokemon that can be walked in the park. The amulet coin is found by walking strait after entering the amity square and you go into the sand castle and it will warp you to another sand castle, go out of the castle walk to you left and then up and the amulet coin is sitting right beside the brownish bench
In Amity Square, which is in Hearthome City. Find it using the Poketch app, itemfinder.
In amity square in the park
Pokemon center top right talk to some one they should have one now I have a lot