Well, after you get all 16 badges, beat Red, and choose your Hoenn starter from him at Silph Co. in Saffron City, go to Pewter City and head north to the museum. Once your there just go and talk to Steven and then go to Silph Co. in Saffron, and talk to Steven and he will offer you a Beldum for a Foretress. The Beldum is the same level as the Foretress you give him, and is holding a Dawn Stone.
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Saffron city. Bring a Forretress because Steven will trade one for a Beldum and that is A SWEET deal.
You can find Beldum in the Safari Zone in "HeartGold" or "SoulSilver" in the Forest area after you have placed 21 Peak Objects in the area and after you have waited 70 days since you added the area to your current Safari Zone customization, additionally you can get Beldum in the event-only course for the Pokéwalker which is called "Winners' Path" (which is still available for download via Wi-Fi Mystery Gift until June 25, 2010) or you can trade for a Beldum from Pokémon Diamond, Pearl or Platinum.
I Will Trade You A Beldum
After you get your Hoenn starter and speak to Steven in the Pewter Museum, Steven will be in the Silph Co. bottom floor. He has a Beldum with a Dawn Stone to trade for a Forretress.
you will find beldum in the cave