Beldum:
Because Wallace is the champion, you must fight Steven. Steven is inside Meteor Falls.
When you enter, go up the waterfall and into the door. Next, go down the ladder, then go
to the ladder in the south. Now, go inside the nearby door. Steven should be to the right.
REMEMBER: If you are going to battle him, you can defeat him with Water, Ice (for
Cradily), and Fire types for most of his Pokemon. He has Pokemon over level 70. .After you
defeat him, go to his house in Mossdeep to get the Beldum.

He's right and took my answer, AGAIN!!!!

In Sapphire, beat Steven and then went to his house and hey presto, a Belsum, BUT! Leave
one room for the party.

After you beat the elite four. go to Steven's house and there will be a poke ball on the
desk. Inside is beldum and leave one room for the party to take it.

All you have to do is beat the elite four go to steven's house and take the pokeball on
the desk leave one room for you to take and oyu don't have to beat steven