In addition to what has already been mentioned, there is:
https://forum.enlightenedstates.com/t/inner-strength-1-0/9128
The Fearless tag, though not in-stock at the moment has a willpower field:
Additionally the Thor Redux tag in the sapien shop (the sapien shop is currently closed however) by my estimation has some type of willpower component. You could also try the Thor mandala on the Sapien Medicine instagram. Also, you might want to consider the Productivity Mandala on the instagram page as well. While it doesn’t strictly affect “willpower” I feel like it just gets you in the mind set to accomplish tasks in an efficient manner which could be very beneficial depending on what you’re looking for.