Where to find Riolu in Platinum?

No, go to Joliberges, by the port of this city there is a sailor, talk to him and he will take you to Iron Island, then go up to the mountain where there is a tunnel, enter and you will find two other tunnels, go into one of them until you reach the elevator at the bottom of the room (downstairs), go down and enter a tunnel where you will find someone who will ask you to follow him, after all the battles against trainers, he will give you a Riolu egg (you must have a maximum of 5 Pokémon in your team).

iron island

You need to take the boat in Rivamar and it will take you to Fer Island, it's that simple. Otherwise, if you want more information, go on Google and type pokébip, you will have all the answers you want.
See you!

Thank you all, I got it and it evolved into Lucario and it is level 100, and I changed its name, before it was shiny Arceus.

you have to beat Team Galaxy and they will give you the egg.

I don't remember too well, but I think you also have to beat Team Galactic.