First go to island 5 and find you way to the memorial pillar. You need to surf and once you find the memorial pillar go to the right of the island and go down and it is sitting by the shore
in sixth island south near waer you get the sapphire
a magnimite has one
you can find it on the island with the big grave and the guy who says his onix is buried there. either he gives it to you or you find it there.
in a gym
You have to give it a metal coat and then trade it.
If you are referring to the original Pokemon Red for the GameBoy, then you can't get a metal coat due to the fact that Steelix or Scizor were not Pokemon back then. If you were referring to FireRed, I have no idea.
You will find one on the Memorial Pillar on Island 5.
It evolves two pokemon: Onix and Scyther. Have each hold the Metal Coat then trade them to a friend take them back and you will get a Steelix and Scizor.
Scizor can be obtained by trading a Scyther while it holds the item Metal Coat.
Don't need to because you can get more from leafgreen or Pokemon colosseum and Pokemon xd gale of darkness.
Trade Onix with a metal coat to someone eles then it will evolve thne ask for it back ,and you have a new Pokemon
Scyther does not evolve by leveling up in firered, It evolves by being traded while holding the metal coat.
Actually you can get many metal coats you can receive 2 in leafgreen and firered and you can constantly buy them in Pokemon colosseum with your pokecoupons.
Same way you get it in any other game: Give a Metal Coat to an onix and trade it.
i dont know if you can get a metal coat...if you get one you can trade an onix with one and get steelix