While Lucario is a powerful Pokémon, it's not found in the wild in this game. You can, however, obtain a Lucario by evolving a Riolu, which you can get through the following methods:
* Trading: You can trade for a Riolu from another player.
* Breeding: You can breed a Riolu by pairing a male and female Riolu, or by pairing a male Riolu with a female Pokémon that can learn the "Egg Move" "Charm" (like a female Gardevoir).
Once you have a Riolu, you can evolve it into a Lucario by leveling it up to level 30 during the day.
Let me know if you have any other questions about Pokémon SoulSilver!