Ledian is a Pokemon from the Johto region, but it doesn't evolve in SoulSilver. The only way to get it in that game is through trading from another game where it can evolve.
Here's what you need to know:
* Ledian evolves from Ledyba.
* Ledyba can be found in the following locations:
* Route 35: This is the most common place to find Ledyba.
* Route 36: Another good spot to find Ledyba.
* To evolve Ledyba into Ledian, you need to level it up to Level 18.
Remember that you cannot evolve Ledyba into Ledian in SoulSilver, so you'll need to trade a Ledian from a different game.