Evolution Item Basics
1. A lot (not all) of the Kanto pokemon that evolve into Johto pokemon will use what're called evolution items. These are new to the update.
2. These evolution items will be used in conjunction with candies to evolve the pokemon
3. There are 5 types of evo items and 8 pokemon that use them (see next section).
4. They are single-use
5. They can only be obtained via PokeStop
6. They appear in your items menu (as pictured above) and take up an item slot until used (more of a reason to take advantage of the deal and buy the extra storage if you can).
7. You can see if a pokemon requires one of these by looking at their profile, where a little picture of it will be next to the candy icon. No picture, no item required.
Evolution Items and the Pokemon they Apply to
Metal Coat: Scyther and Onix
King's Rock: Poliwhirl and Slowpoke
Sun Stone: Gloom and Sunkern
Dragon Scale: Seadra
Upgrade: Porygon
Note: Golbat is an example of a pokemon that doesn't require an evolution item, even though Crowbat is a Johto pokemon. They don't all require an item. Also, Espeon and Embreon are acquired by following a naming convention, Sakura and Tamao, respectively.
I will update as soon as I get any info on frequency and chance rates. As far as I've experienced, I've only gotten one evo item in a night of playing, for your reference.