It's possible you're thinking of something else, like:
* The Flag Pokémon: This would be the Emblem Pokémon that you get from the Galactic Headquarters in Veilstone City. It's a special item that lets you access the Symbol of the Union in the post-game.
* Flags as decorations: You can't obtain flags to decorate your house or rooms in Pokémon Pearl.
If you have another question about Pokémon Pearl, please let me know!