Just keep on breeding/searching for one. There is a 1/8192 chance of any Pokemon being shiny. You can increase this chance by getting the Shiny Charm from Professor Juniper after completing the National Dex (BW2), as well as by breeding a female Riolu/Lucario with a foreign Pokemon in a technique known as the Masuda Method (works in Gen IV and up).
You can also always trade/hack for one. Go on our chat and perhaps someone there will have a shiny Riolu up for trade.