For me, therapy has been decades of trial and error. I started right after I left home, almost 30 years ago. The first therapists I saw seemed to be completely clueless about NPD. I actually had one who said to me, "Now really, why would your mother do those things to you?" I told her, if I knew that, I wouldn't be sitting on her couch.
Most of the early therapists that I saw were one-time visits. I never returned because I knew they just weren't going to work. I gave up for several years. About 15 years ago, I developed Generalized Anxiety Disorder, and part of my treatment was to see a psychiatrist. That woman messed me up REALLY badly. This is the one who accused me of being a chronic over exaggerator and "drama queen." At the time, I didn't realize how badly she was hurting me. After several years of this, I actually started stressing out over having to go to my monthly appointments, or would come home stressed and upset over something she said. So I stopped seeing her.
My current therapist is the closest I've come to a match. After almost 30 years, she was the first one to say the words Narcissistic Personality Disorder. She totally believed what I told her about my NM, so I felt a huge relief to have found her. That said, there have been times when I've not agreed with her methods, specifically, advising me to support my codependent father. Though in recent weeks, F has done some very hurtful things to me, and she's now back on my side, so to speak. But for a while, I was considering dropping her. In fact, I had started a thread about it. I think it was called "Can a therapist do more harm than good." If you go back a page or two you can probably find it.
I plan on moving back home to CA end of the year, which will force me to find another therapist. So while the current one is working, I'll have to be moving on. Even though we have had our disagreements, I feel that she has helped me, and that I'm better off for having a therapist than not. She has a lot of patients who are victims of Ns, so she knows quite a lot about the disorder. That makes all the difference. If you end up with a therapist who isn't all that knowledgeable, they won't "get" what you're trying to say. I honestly don't know how to search for someone who is knowledgeable in that area, but maybe look for someone who specializes in family counseling. Odds are, someone who deals with family issues will have experience with Ns.