How do I get evolution items like King's Rock?

How can you obtain the new evolution items like King's Rock after the new update?

As mentioned in the official Gen II release announcement, you are able to get these evolutionary items from PokéStops, just as you would an egg. While the natural drop rate of these items is quite low, you are guaranteed to receive one on your 7th day Pokéstop streak.

Be on the lookout for new Evolution items at PokéStops, which you’ll need to evolve some Pokémon.

Pokémon whose evolutions require these items are:

• Seadra → Kingdra (Dragon Scale + 100 Horsea Candy)
• Slowpoke → Slowking (King's Rock + 50 Slowpoke Candy)
• Poliwhirl → Politoed (King's Rock + 100 Poliwag Candy)
• Onix → Steelix (Metal Coat + 50 Onix Candy)
• Scyther → Scizor (Metal Coat + 50 Scyther Candy)
• Gloom → Bellossom (Sun Stone + 100 Oddish Candy)
• Sunkern → Sunflora (Sun Stone + 50 Sunkern Candy)
• Porygon → Porygon2 (Upgrade + 50 Porygon Candy)

It might be worth mentioning that while Blissey and Crobat are newly-available evolutions of Gen I Pokémon, these two do not require special evolutionary items to evolve.