Hi Sad Momma, welcome to the group
glad you decided to reach out.
It's good that your D18 is open to continuing therapy -- that's huge! Definitely a positive that after RTC/inpatient/meds/etc, she sees the value and wants to keep going with counseling.
Can I ask, what was she in the RTC for? Was it for BPD specifically, or a different diagnosis?
You've probably heard about Dialectical Behavioral Therapy (DBT), which is a common treatment modality for those with BPD. If you're not sure where to start looking for a good therapist, you can try the psychologytoday.com site and click "Find a Therapist" up top. Enter your city or zip code and a list will pop up. What you can do then is in the "types of therapy" menu that appears with the other option menus, select "Dialectical Behavioral (DBT)" and the list should narrow to providers with DBT experience.
If you find someone on that DBT list who also has a Ph.D, that can be an indicator that the person has a lot of experience with BPD patients.
Of course, relational fit is also important. You and your D18 can "interview" a few providers to see who she clicks with -- of course, you can also vet the list first down to ~3-4 names that you've already researched. Not everyone who says they do DBT has the same amount of experience, so you can talk to them first and see if it's a good person to add to the list for your D to pick from.
Also, check with D18's current T to see if that T has any recommendations for local therapists with experience with BPD in young adults. While this T may not be the best fit moving forward, s/he might have good connections to share.
Hope that gets you started, and let us know any time how you guys are doing;
kells76