You get one on the boat. When you complete the Elite Four (you need to beat all 8 badges, then swim to the right in New Bark Town and so on... Then pwn Elite Four and the Champion. They have pretty tough Pokèmon), you get a call from Elm, and he gives you a ticket. Go to Olivine, and go to the port. A sailor will ask you for the ticket, and you'll end up on the boat after all. You complete a little quest there (this is another story; albeit it involves finding a girl and stuff), and a guy gives you a Metal Coat as a thanks. The boat will then arrive to Kanto.
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To get scizor in Pokemon gold you must trade a Scyther that is holding Metal Coat. Metal Coat can be attained on the S.S. Aqua after you defeat the Elite Four.
You need to trade an Onix holding a Metal Coat.
onix has to be traded and be holding metal coat
you trade your onix with it holding a metal coat
the Pokemon that evolve with metal coat are steelix and scisor