I am doing my Dream Team in Sapphire on Pokemon Online and I need Heavy Slam but I don't know how much Aggron weighs and if there is an item that raises their weight.
There is no item that makes a pokemon heaver. The only way to increase weight is by the ability heavy metal, which is found on aggron as a DW ability.
Aggron's Weight: 793.7 lbs (360.0 kg)
With Heavy Metal: 1587.4 lbs (720.0 kg)