SKYbrary offers a useful database of accident reports that are useful for CRM case studies. They are summaries, so you don’t have to wrestle through 300 pages of report to find the essence. A full list can be found here (click on external link).
https://skybrary.aero/accidents-and-incidents
Certainly topics on SKYbrary have a reference list with relevant reports, see for example continuation bias.
https://skybrary.aero/articles/continuation-bias
One useful example of a group exercise would be a discussion on critical thinking based on the following case: