Ask around your department. Surely there is a professional journal/magazine, titled e.g. The Theoretical Physicist, that has these blue pages listing conferences of possible interest to members of the profession ?
Every university has a theoretical physics school and all will have seminars. I suppose you need to select those universities whose Schools of Theoretical Physics do research in your area. Create a mailing list of the contact emails for Coordinator of Seminars in each of those Schools. Then send an email to this group asking to be added to their mailing list for upcoming events.
In practice you may only be interested in going to seminars within reasonable travel distance, unless the quality of the seminars is very high/prestigious. That might cull the herd a bit for you !