First of all, the only way to obtain these evolution items is to visit PokeStops and spin the Photo Discs. Items appear randomly, so whether you receive one or not is purely up to chance. Unfortunately, all you can do if you don’t have the necessary evolution item is to keep spinning Photo Discs until one appears.
Once you finally do find an evolution item, you can check this list to see which Pokemon it affects:
- Dragon Scale
- Seadra evolves into Kingdra for 100 candies
- King’s Rock
- Poliwhirl evolves into Politoed (instead of Poliwrath) for 100 candies
- Slowpoke evolves into Slowking (instead of Slowbro) for 50 candies
- Metal Coat
- Scyther evolves into Scizor for 50 candies
- Onix evolves into Steelix for 50 candies
- Sun Stone
- Gloom evolves into Bellossom (instead of Vileplume) for 100 candies
- Sunkern evolves into Sunflora for 50 candies
- Porygon evolves into Porygon2 for 50 candies
As of the latest update, these are the only eight evolutionary lines affected by the evolution items. In the original games, several other Pokemon currently available to catch in Pokemon Go also required the player to find an item in order to evolve them, but, for the time being at least, Niantic is only going to make players use the evolution items to acquire certain Johto-region Pokemon.