Hello Helena123, glad you found the group.
If you're on the east coast, have you already heard of McLean Hospital in Mass? They have age-separated mental health programs and are generally well known and well respected for their BPD program. For starters, you can check out their teen-to-young adult DBT specific program here:
https://www.mcleanhospital.org/treatment/3eastI would hazard a guess that even if McLean is not the ideal fit for your child, if you called them up and described your situation, you may be able to get suggestions of other centers.
Hope that's a helpful starting point;
kells76