There is no other way to evolve Happiny in Pokémon Pearl. The Oval Stone is the only item that can trigger its evolution into Chansey.
Here's what you can do:
* Start a new game: This is the easiest option, as you'll be able to obtain the Oval Stone again during the normal course of the game.
* Trade with a friend: If you have a friend playing Pokémon Pearl who has an Oval Stone, you can trade for it.
* Use a cheat device: While this is not recommended, if you're playing on an emulator, you can use a cheat device to give yourself an Oval Stone.
Remember, using cheats can potentially affect your enjoyment of the game, and it's important to play fairly.