Finding ideas for investigative reporting

Investigative reporters turn routine observations, reports, tips and conversations into questions about the way the world works. Curiosity is the starting point for great stories. 
A Poynter coffee break course lists some places to begin. Check it out here:  
Two-minute selections from some Poynter News University online courses are offered on as coffee break classes. More than an an overview of the full course (courses for which tuition is charged) the shorts offer some good material and suggestions.