How do we get special items for certain evolution?

pokestop random. i think its about 1/2000 chance per evolution item.

Got a Metal Coat during my 7th day streak spinning a PokéStop.

No one has played the game long enough to accurately assess how common special items are, but they're thankfully not common. Just keep spinning those pokestops.

Got a Kings Rock this morning. I'd estimate I've hit 70 poke stops since Gen 2 before that item dropped.

Now to decide on Politoed or Slowking..... and to use it on a pretty good mon, or hold and wait for a near perfect mon. At least it's nice to have one to need to make decisions on. Good luck on your Pokestop spins!