Starting at level 10, Metal Coats can be rarely obtained from
Spinning PokéStops or Gyms
Opening a Gift
While normally rare, from March 21, 2017 onward, the player is guaranteed to find an Evolution item when obtaining the bonus for spinning a Photo Disc for the seventh day in a row.
Additionally, the player receives a Metal Coat from these Special Research tasks:
"A Ripple in Time" (Part 4)
"Let's GO, Meltan" (Part 6)
Hope this helps. Also, to evolve Scyther, you need:
Scyther requires 50 Scyther Candy and a Metal Coat to evolve into Scizor.