Sabrina, the Psychic-type Gym Leader, is located in Saffron City in Soul Silver. However, you won't find her in the gym building.
Here's how to find Sabrina in Soul Silver:
1. Enter Saffron City: You'll need to have obtained the Silph Scope from the Silph Co. building in the city.
2. Talk to the Man on the Street: Once you're in Saffron City, a man on the street will tell you about the "strange happenings" at the Silph Co. building.
3. Enter the Silph Co.: Head to the Silph Co. building and make your way to the top floor.
4. Find Sabrina: Sabrina will be waiting for you on the top floor, where she'll challenge you to a gym battle.
Note: It's important to note that Sabrina is NOT in the gym building in Soul Silver. She's located in the Silph Co. building, which is a separate location in the city.