Journalists are increasingly investigating and reporting on problematic online content such as misinformation, disinformation, and conspiracy theories, leading to the creation of a new misinformation beat. The process of collecting, analyzing, and reporting on this kind of data is complex and nuanced. It is especially challenging as online actors attempt to undermine their work. Through in-depth interviews with twelve journalists, we explore how they investigate and report on online misinformation and disinformation. Our findings reveal some of the unique challenges of reporting on this beat, as well as the ways in which reporters overcome those challenges. We highlight and discuss how journalistic values could be better embedded into the design of tools to support their work, the power dynamics between social media companies and journalists, and the promise of collaborations as a way to support and educate journalists on this beat. This work provides contextual knowledge to researchers looking to better support investigative journalists - on the misinformation beat and beyond - as their work becomes more entangled in sociotechnical systems.