On iron island's last room before the exit way in the back. go up in the empty room and you can find metal coat in the middle.
You can use it on a steelix to trade, hold it, and evolve it
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Catch a Scyther. Then get a Metal Coat. Attach that to Scyther and then trade.
When you find the man's missing granddaughter on the S.S. Aqua then entertain her by playing hide and seek the man will give you the metal coat. You can only do this after beating the elite 4.
breed an electabuzz if you want an elekid in Pokemon platinum
trade from firered and on firered you have to evolve scyther (which you can catch in the Safari Zone) by trading it with a metal coat
You can't find Vibrava on Pokémon Platinum