An open minded investigator can find a problem in many places that are scarcely thought about, let alone reported on, like how well maintained is a local airport's tarmac?
How well staffed is your state or local health department, and what information do they keep track of?
Private Sector Service Industries
Visit several local businesses and ask a few questions, this might be the best source of information on a controversy.
Another example of a sound research lead could be the vast discrepancies between the costs of wedding cakes at grocery stores versus private bakeries, and whether the extra cost is justifiable.
Or, are there any laws governing particular private industries in your city or state that seem unfair?
Public Services or Utilities
In a city one author lived in, she learned that the city's sewer system was in dire straits and might already be polluting local ground water. Public services and utility companies are often a solid source to explore for an investigative report.
How many people truly understand how an electric or garbage company operates? Simply finding out what happens to profits or losses made by these companies would enlighten many readers.
Never underestimate the impact a truly original essay topic can posses. Investigative essay topics about popular fads, like tattoos or body piercings and their possible side effects could be interesting.
Find out if there have been any UFO sightings in your area and how reliable the witnesses seem.
The habits or impacts of homeless people in your city may make for an out of the ordinary, investigative, essay topic worth exploring.