You can trade with Steven in-game to get one. You will need to have done the following before you can trade with him.
Receive the Hoenn starter from Steven at Silph.Co, which is located at Saffron City.
Talk to him at the Pewter City Museum.
Go back to Silph Co and talk to him, he will then offer a Beldum in exchange for a Forretress, the evolved form of Pineco.
The Beldum will be the same level as the Forretress you traded him, it's nickname is Iron and it holds a rare Dawn Stone as a bonus. Hope this helps :)