One form of protection for both you and your witness may be a legally-recognised, signed, witnessed statement: an affidavit. Important people often have large egos, and the same personality traits that makes these people individualistic and bossy in dealing with you, may be the traits that possessed them to ‘stick their neck out’ and come to you with information.
This kind of legal statement, initialled on every page and signed in the presence of a lawyer, is acceptable to courts in most countries and has important legal consequences. It signals that your source is willing to appear in court and provide evidence, if required. An affidavit should be given to a trustworthy lawyer for safekeeping. If your story results in legal challenge or action, the existence of the affidavit makes clear to anyone challenging it that your source is prepared to reveal him or herself if offered the protection of the court. It also protects you if your source later retracts the story.