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 Pokeball down there containing a Prism Scale.
It can also be found as a hidden item in Pokemon X and Y in Couriway Town. It is loced 10 tiles behind and 2 right of the man who sells fresh water. It's at the single grass tile surrounded by tiles shallow water. The preferable way to find this would be to use the Dowsing Machine given to the player earlier in the game.