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