Question

Where can I find a Metal Coat in S/M?

Answer 1

There's no clear cut method in S/M that will guarantee a Metal Coat. However, it is found:

Held by wild Magnemite (5% chance), wild Skarmory (5% chance), and wild Beldum (5% chance)

Meaning you'll have to catch these Pokemon and you might get lucky sooner or later.

Comments

I tried this method before. It took almost an hour or two until I finally Thieved a Metal Coat from Magnemite.

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Once I caught a Skarmory with Metal Coat first try.