I found several agencies that do this.
The interesting thing about googling “self defense certification” is that many of the courses are online. This doesn’t seem particularly useful, in my opinion. As you know as a martial artist, hands-on practice for most techniques is critical because you have to have the muscle memory and practice touching people a variety of heights and sizes to figure out where pressure points are and how to hold, push, twist, etc. I have a black belt in taekwondo, which included lessons in self defense and one-step sparring at each belt level, so I have experience in self defense but would not consider myself qualified to teach it.
This particular thread was posted a few years ago. It appears that at least one of the respondents does live training for self defense instructors.
I think, the easiest way is to take a online course.