There is no Heavy Metal item, its a ability that increases weight. Aron, Lairon, Aggron, Bronzor, and Bronzong have this ability.
I assume that you mean the Iron Ball. If so, then you can buy it in the Gear Station for 48 BP. You can also get it through the Ability Pick Up. Level 31-50 Pokemon have a 1% chance of finding it. Source: http://serebii.net/itemdex/ironball.shtml