I take some great continuing education courses through www.exerciseetc.com. They do have a number of different level stretching courses, so it may be worth checking out their content. Most of what I saw was home-study, so I’m not sure if that is what you are looking for, or actual live workshops. Their credits are accepted by many certifying agencies, including NASM, which I see you have.
Also, hopefully you have checked the IDEA site for some ideas. Hope this helps you.