While there are items that affect PP, like PP Max, they're not available until after the Elite Four.
However, you can still increase your Pokemon's PP in a few ways:
* Level up: As your Pokemon level up, their PP will increase naturally.
* PP Up: This item increases the PP of a specific move by 10 points. You can find it occasionally from trainers or as rare loot in certain areas.
* PP Max: This item increases the PP of all moves a Pokemon knows to their maximum. It can be obtained from the Game Corner in Goldenrod City after you beat the Elite Four.
* Certain moves: Some moves have a higher PP than others, so you might find that certain Pokemon are better suited to some attacks.
Remember, there's no "easy" way to get PP Ups in Soul Silver. Your best bet is to focus on leveling up your Pokemon and using the items you find during your journey.