The coolest thing about the internet is the vast amounts of niche knowledge available in an instant.
Some knowledge can come from people's personal blogs who work a very unique job and also share that online. But the amount of people who have a unique job is quite small and the subset of people who write about what they do is even smaller still – so this type of blog is few and far between.
Another good way to find niche secrets online is to follow the footnotes. Traditional media companies sometimes write about what's going on in the world and will link to an original source. That original source likely contains much more fascinating insights than the piece that is synthesizing what they say. Try reading the original piece. This is also a good habit to get into because you learn how to shape ideas for yourself instead of relying on someone else to tell you what to think.
When you discover a niche secret, combine that with something you already know, remix a story from history, and you now have a new idea, complete with an illustration to use as an example that you discovered and synthesized all alone. How cool!