Hi Julien Boulder, welcome to the group. I'm guessing you joined because your partner has BPD type traits?
While I'm not CA based, fortunately Psychology Today has a practitioner finder link, and there's a sublink specifically for the LA area:
https://www.psychologytoday.com/us/therapists/ca/los-angelesIf you click on the "types of therapy" drop down, you can pick DBT (Dialectical Behavioral Therapy -- a modality intended for those with BPD) if you want, as that might narrow the pool of providers to those with experience with BPD. Gottman Method is good for couples, though I believe it's intended for "broadly normal" conflicts vs personality disorder based conflicts.
I'd recommend "interviewing" more than one professional and asking some targeted questions about their experience. It's normal to not go with the first person you land on (though it can happen!).
Keep us posted on if that helps you find a couples therapist. I agree with you that experience is key. BPD-based relational conflicts are not for beginning therapists.