There are two subjects here, finding a therapist for your significant other, and finding a therapist for yourself.
Even for yourself, it is important to find a therapist who is familiar with BPD, and has worked with the family members of borderlines before. The disorienting, lie-filled world of the Non is confusing for therapists who haven't been down this road before. Many therapists in couples therapy get confused about who the sick one is, and end up doing more harm to the Non's self-image. For this reason, you might consider getting a therapist for yourself, who does not even know your significant other. This is a much safer way to investigate your own condition.
-Kelly