I think the biggest thing is I would make adult attachment therapy an actual side specialty to attachment-based therapy for kids and teens. A lot of therapists think they do attachment therapy, but they really don't. It's not well-defined. The best thing you can do is to ask for references/referrals to find someone who is known in the field to do specifically what you are asking for.

In looking for C, I emailed other practices and specifically described my situation and my needs (I wanted someone with experience working with adults on attachment, and I need between-session contact). They knew of C and referred me his way. I feel a lot more confident working with C because of his long-standing history and recognition in the community for doing this kind of work. He doesn't hide that he does this work; I am not an experiment to him.