Ball Capsules were introduced in Pokémon Black & White, so they aren't available in older games like Pearl.
If you're looking for ways to store Poké Balls in Pearl, you can use the following:
* PC Boxes: You can store Poké Balls and other items in your PC boxes, which can be accessed at Pokémon Centers.
* Bag: You can carry a limited amount of Poké Balls in your Bag, which is accessible anytime.
While you can't use Ball Capsules in Pearl, you can use the above methods to keep your Poké Balls organized!