Final answer:
Threat intelligence researchers should monitor OSINT, the dark web, and vulnerability databases.
Step-by-step explanation:
Threat intelligence researchers who search for leaked credentials should monitor OSINT (Open-Source Intelligence), the dark web, and vulnerability databases. OSINT refers to information that is publicly available and can be gathered from various online sources, such as social media, forums, and news articles. The dark web is a part of the internet that is not indexed by search engines and is often associated with illegal activities. Vulnerability databases contain information about security vulnerabilities in software and systems.