Sorry but you can't get them on Pokemon Platinum. You have to migrate regi rock regi steel and regi ice from eiter ruby or sapphire, ( idk about emerald) i hope i helped ;)
Ice and Rock Pokemon
Steel-Type attacks deal 2X to Ice- and Rock-Type Pokemon, and 0.5X damage to Fire-, Water-, Electric- and other Steel-Type Pokemon.
Shieldon is a Rock and Steel type pokemon.
Aron is a Rock and Steel type pokemon.
Arron is a STEEL and ROCK type pokemon.
Lairon is a Rock and Steel type pokemon.
Aggron is a Rock and Steel type pokemon.
Probopass is a Rock and Steel type pokemon.
Steel is super effective against rock type pokemon.
Rock type Pokemon are weak against Water, Steel, Grass and Fighting.
Fire, Fighting, Steel and Rock.
Fire, fighting, rock and steel.