How do you get Beldum in Pokemon Diamond?

Answer 1

Go talk to Lucas'/dawn's little sister and she will tell you the herd that's swarming and where on that particular day. One of these days, Beldums will be swarming somewhere on route 229 (the sandstorm part) and you can go catch one. Beware though cause the Beldums only know the move take down so if you don't catch it in a certain time, it will kill itself. And also, for some odd unexplainable reason, Pokemon have chosen to make Beldum's catch rate the same as raquaza, Uxie, groudon etc etc. really odd. I still have yet to catch one : / ~Sherry
Beldum is a swarm Pokemon, meaning It could be found only when Dawn's sister tells that it's the swarming Pokemon of the day. Dawn's sister house is in Sandgem Town.

You may also get it from the GTS if it's recorded in your pokedex that you've seen it. You can also get by transferring it from Pokemon Ruby/Sapphire/Firered/Leafgreen in the PalPark.

You also get one from emerald later on in the game. There is this place, I forgot the name, it has twin gym leaders, it's a physic type gym. Well anyway, there is a house near the pokecenter and it has a pokeball and its a lvl 5 Beldum. If i were you I would train it because it can learn good moves. Mine is lvl 100 and it knows earthquake, brick break, physic, and meteor MASH. Have fun :D