Suicune is exclusive to Pokemon Pearl, where you can catch it after completing the main story.
Here's how to find Suicune in Pokemon Pearl:
1. Complete the main story: You need to beat the Elite Four and become the Champion.
2. Find the Lake Guardians: After becoming Champion, you'll need to find all three Lake Guardians: Uxie, Mesprit, and Azelf. They are located in different locations throughout the region.
3. Go to Lake Acuity: Once you've found all three Lake Guardians, Suicune will appear in Lake Acuity, the location of Uxie.
4. Catch Suicune: Suicune is a high-level Pokemon, so be prepared for a tough battle. Use your best Pokemon and be sure to have plenty of Poke Balls!
Remember, Suicune is only available in Pokemon Pearl, not Pokemon Diamond.