How do I get Steelix?

There are three ways to get Steelix.

The first way is by evolving your Onix which can only be done by trading your Onix while holding the item Metal Coat.

The second, easier way is by trading any pokemon to Gentleman Preston inside the Olivine Gym in the Johto region.

Then you can go the old fashioned route and farm them.  Steelix can be found at Mt. Silver in the Chamber (a rare spawn) or at Cliff Cave in Johto (also a rare spawn.)