You may gather evolution items from PokéStops, but evolution items are not general-purpose items like berries or potions. They're specific to certain types of Pokémon. If you want to use a Sun Stone, for instance, there are two specific Pokémon that need it to evolve. Here's everything you need to know about them.
Pokémon Go Sun Stone Evolutions: Meet Bellossom and Sunflora
There may be other Pokémon Go evolutions in the future that require Sun Stones. But for now, Sun Stones are only required to evolve Bellossom and Sunflora in the mobile game, according to the Silph Road.
Bellossom is a possible evolution of the Gen 1 Pokémon Gloom, which is an evolution of Oddish. If you evolve a Gloom without a Sun Stone you get the Gen 1 Pokémon Vileplume, but if you have a Sun Stone when you evolve the Gloom, you get a Bellossom.
Bellossom is a grass-type Pokémon. According to its Pokédex entry, it doesn't offer particularly strong stats in the main games, though it's possible it could get some sort of boost in Pokémon Go. Even if it's not the best fighter, you'll still need one to complete your collection.
Sunflora is a Gen 2 Pokémon that evolves from another Gen 2 Pokémon, Sunkern. In this case, there's no alternate evolution for Sunflora if you don't own a Sun Stone. According to its Pokédex entry, Sunflora can convert sunlight into energy and is only capable of moving around during the day. It's a grass-type with decent stats and a particularly strong Special Attack, though these main game stats may not carry over to Pokémon Go.
Remember, it appears as though evolution items are given out on the seventh day of a seven-day PokéStop streak. You may not get a Sun Stone the first time you try this, but keep at it and eventually you should find the item you need.
Check out more Pokémon Go guides, tips and tricks
Be sure to check out Mic's guides on how to get stardust, how to determine how long it will take you to reach level 40, the kind of Pokémon you get from 10-kilometer eggs, how to create new PokéStops, how to maximize your chances of catching Pokémon and how PokéStops distribute Pokémon eggs. Also check out how to catch Gen 2 baby Pokémon, our analysis of post-balance update Chansey and Rhydon and everything you need to know about finding the long-awaited Pokémon Ditto.