Private investigators do not reveal their sources. However, since we are in this business we do know that many Investigators and researches use a combination of public and private databases to conduct their investigations and research. Typically, the data they obtain can only be accessed by accredited companies or licensed individuals.
These sources often originate from public records sources, government agencies based on the freedom of information act, credit and telecommunications providers, magazine subscriptions, 3rd party private marketing and data aggregation firms, voter records, sweepstakes entries, web scraping, social media profiles, and many others. The information is sometimes culled manually to ensure the most recent data is retrieved. All data is provided as-is ,which means it is not verified unless the search description specifically states it is.