I have SS, and I can't find a clue of how to get a Lucario. And I really want to breed it to get a Riolu.
Unfortunately there is no way to find a Lucario or a Riolu is HG/SS. You'll have to trade with a friend, or get one on D/P/P and trade it to your HG/SS.
See here for a video guide of how to Riolu on Platinum version.
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