This page has the details about the Shiny Beldum distribution that is taking place after the release of Pokémon Omega Ruby and Alpha Sapphire.
From November 21, 2014 through January 14, 2015, you can download a Shiny Beldum in Pokémon Omega Ruby and Alpha Sapphire. The Beldum is holding Metagrossite, which is the Mega Evolution stone that will allow Metagross to Mega Evolve into Mega Metagross. The Beldum knows Hold Back, Iron Head, Zen Headbutt, and Iron Defense.The Beldum's original trainer is Steven.
How to Download
Make sure that your 3DS can connect to the Internet. Then after you have saved the game, you can go to the main menu and choose Mystery Gift, then choose Receive Gift, then choose Get via Internet. After you download the Beldum, go to a Pokémon Center in the game and talk to the girl in front of the desk to receive your Beldum.