I would quite like to use a Metagross as my tank in Pokémon Soul Silver. Is there anyway I can get a hold of this Pokémon or any of its pre-evolved forms without importing them from another game?
Yes, there is, but you're probably not going to like the alternatives. You have 3 choices.
The only way to get Metagross is to evolve from Beldum or Metang.
Note that all of these options require that you've defeated the Elite Four first. There is no way to use a Metagross, even including trading, before you've defeated the Elite Four and gotten the National Dex.