Wild Pikachus hold Light Balls. I think it's like a 5%chance or something. It's an item that doubles Pikachu's Attack and Special Attack, and breeding a Light Balled Pikachu (sounds dirty, huh?) with a normal one you'll get a Pichu with Volt Tackle. More stones can be acquired by mining them in the Underground. Also, correct me if I'm wrong, but I think there's someone who trades evolutionary stones for colored shards. I don't have Platinum, so I'm not sure about this, but I think I read that somewhere.