1. The Internet Has Limitations
It turns out that the editors at Wikipedia aren't nearly as interested in music history as I am. Articles on music history are often short and incomplete. Certainly, you can start with a google search on most topics, but don't count on getting any in-depth material. Instead, let google lead you to print sources and help you find books to read (even then, you should still make like Hermione and go to the library).
2. Ask the Experts
Music history professors are often pretty approachable. In fact, I once ran into one at a book store when I was researching an article. I asked her advice, and she pointed me to some excellent books that have proven very helpful. Even if you're not in school, as I'm not, you could reach out to local experts. Most professors have their email addresses online, and while they may be too busy to help you, they may also be thrilled to meet someone interested in their research.
3. Listen to the Music
If you're researching Marin Marais' suites for viola de gamba, it's not enough to read about them--find a CD or look them up on Spotify and listen to them (ideally with a score in hand). Again, the internet can disappoint you (the people at Spotify or youtube are probably about as interested in music history as the editors of Wikipedia, maybe less so). However, many libraries (even standard public libraries) have CDs you can check out, and music libraries may have an extensive selection of classical, Medieval, and Renaissance CDs. Now, whenever I go to the library, I check out CDs as well as books. It's a free way to find great music to listen to, and helps my research.
4. Look at the Bibliography
Let's say you find a great article on 19th century trombones, but it just doesn't have all the information you need, and you don't know where to find more. Go to the end of the article, and check out it's bibliography, or the list of articles/books cited by the original article. Those are a great place to continue your research.
5. Go to the Library
When I was a graduate student, I had the opportunity to work in the music library at my school. One of the most useful parts of my training involved learning to use all the online library resources, from JSTOR to WorldCat. There's vast resources available in most academic libraries, and many of those are open to the public.