The Prism Scale (Japanese: きれいなウロコ Beautiful Scale) is an evolutionary item introduced in Generation V. It is a mysterious scale which shines in rainbow colors. When a Feebas is traded holding the scale, it will evolve into a Milotic.
The Prism Scale can be found on Route 13 in Pokémon Black and White and in Pokémon Black and White Versions 2. Talk to the Pokéfan standing to the left of the computer in the Poké Center in Undella Town and they will give you one.
You can also find it in Route 1 in Black and White 2 by going to the body of water and surfing until you hit land, it should be under the screen. Then, battle the trainers there and then go around the gate and the sign that reads "Route 1", past the male Pokémon Ranger then go down the small pathway and there should be a Poké Ball down there containing a Prism Scale.
It can also be found as a hidden item in Pokémon X and Y in Couriway Town. It is located at the single grass tile surrounded by tiles of shallow water in the area behind the man who sells fresh water (10 tiles behind and 2 right of the man). The preferable way to find this would be to use the Dowsing Machine given to the player earlier in the game. Another way to get a Prism Scale in X and Y is to go into the building across from Café Ultimo in Lumiose City, go up to the fourth floor, and talk to the fairy tale girl while wearing an outfit of her liking.