Question

Where can I find a Beldum, or one of its evolutions?

Answer 1

When there is a swarm of pokemon on route 228 there is a 40% chance of a beldum appearing and you can't find any of its evolutions in diamond

Comments

Oh okay. what about sapphire?

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In sapphire, go to steven's house in mosdeep city after becoming the champion, and there is a pokeball and letter for you, the letter says the beldum is a gift for you.

Answer 2

In saphire, Steven will give it to you after you beat the league. He is in his house at the seventh gym town. (What was the name again)
Zekrom is right about Diamond. Beware, they are very hard to catch in d/p/pt because they are found at levels over the fifties.