I attended ALPA's Basic Safety School in the US, the Aircraft Accident Investigator's Course and the Advanced Aircraft Accident Investigator's Course, all excellent and well-taught.
Another sourse is here in California at the University of Southern California, which has its own Air Safety department, and the Southern California Safety Institute, which also teaches accident investigation.
Hope you find something to your liking. Feel free to PM me with your e-mail, and I'll send you what I have.