One of the biggest draws of gen two in Pokemon GO are the fancy evolutions that require special items, such as the Metal Coat or Dragon Scale. Some players vying for these items are having a hard time actually finding them, though.
As far as anyone can tell, evolution item drops are random. There are many reports of people obtaining evo items during a seven day Pokestop streak, but it isn't guaranteed. Some players report getting evo items outside of streaks, some say they haven't gotten anything at all in spite of playing every day since the update launched. Personally, I haven't found anything yet, and I'm not alone.
Pokemon GO Hub (via Trainer Tips) decided to compile some data relating to Pokestop drops, and the numbers are kinda eyebrow-raising. Out of a recorded 23,700 item drops, they found that evolution items appeared less than a per cent of the time, combined:
Now, obviously, the sample data is a drop in the bucket compared to the number of players out there using Pokemon GO, but anecdotally, this seems to track. And really, it makes sense for evolution items to be rare, because it creates a roadblock for players to just obtain everything in the update from the get-go; you'll have to keep spinnin' those stops. Even so, the rarity of evolution items are a thin, unsatisfying way to give an already scarce game more longevity. Pokeballs, meanwhile, are plentiful and abundant. You can read the stats for all the item drops in this data here.
When reached out for comment regarding evolution items, Niantic declined to share anything. For now it seems safe to say that if you want to catch 'em all, you'll need some patience!