Question

Where can I get beldum in ORAS?

Answer 1

To my knowledge, and the facts back it up, it is not possible to obtain Beldum in the wild.
The only way to get it is to get *Steven's Beldum.
(that is if we are talking in game*, you could always try with the PSS)

here is how:
https://www.youtube.com/watch?v=agbo_T6GikU
And for more info on beldum and how/where to catch it in other Generations, check this links:
http://www.serebii.net/pokedex-xy/374.shtml
http://pokemondb.net/pokedex/beldum

Hope I helped!

Answer 2

After the Elite Four, if you go to Steven's house in Mossdeep City, there will be a pokeball on his desk. You'll see a note from him that the Beldum (in the pokeball) is for you. This is the only Beldum you can get in the game. Unless you breed or get one in the Wonder trade (or that event with the shiny one, but I believe it's over)