You haven't made it far then. You can catch old pokemon. Yes, you can catch Riolu in the challenger's cave after you beat the elite 4. He's rare though.
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You Cannot catch lucario on pokemon black the only Way is to trade him or catch a riolu.
You can catch Riolu in Challangers cave. You can't enter challangers cave till you have beaten the eliet four at least once.
You can't catch it in Black, but you can catch it in White. In the White Forest.
you have to get in Pokemon white
not catch but by Pokemon transfer or the Dreamworld