I need help in my thesis with my scale.

Hi, Im stuck in a tough spot therefore i would appreciate advice regarding the situation im dealing with. There is a scale developed by Micheal J. Fimian ‘Teacher Stress Inventory’ which is perfect for my study considering the target population i have. However the issue is that the author is deceased and there are no co-authors. He passed away in 2021. Furthermore, I can’t find his scale in any of his journals published on the Internet except separately. I have seen researchers use his scale after his death. I have tried to find solution to my problem multiple times and I can’t ask others for help. Reddit is my last hope.
So how will I show proof of permission for the scale for my thesis in this circumstance. Secondly, will the scale be considered in the public or private domain?