Consider starting one! You can begin by asking the following questions:
• Is anyone in the community currently working on youth violence prevention? Are they interested in a more coordinated effort?
• What organizations work on related areas like youth development, health, and school-based intervention? Are they willing to work in a larger group?
Additionally, check out the above for a list of organizations to consider (for example, someone from education or housing). Different organizations will have people with different knowledge and skills.