I write about many things here, but one topic that I like to engage in, and write about is research. Usually my first step is Google Scholar. You can configure it in many different ways, and even provide your school information to help make the process smoother (you typically have to log into many research databases with an ID, which you do through you school log in if you have one). Otherwise, some of these site will require payment (or sometimes they have a certain number of free articles allowed). Google Scholar is also very useful in terms of looking up patents for theater technology.
I also frequently use the New York Times. I was reading a book about a murder in NYC, and it dawned on me while reading the book that the reason I could not get earlier information about technical direction, technical theatre, and such from the times (late 1890's I think is the earliest that I have), is because they were not the dominate newspaper in NYC before that time. The others that were are not as easily searchable, but I have not spend a significant amount of time following those leads. I will say that newspapers and trade magazines are helpful in terms of tracking down industry trends and shifts. For instance, many of the large scene shops switched hands over the years. It is published that Variety was purchased by Showman Fabricators, but following the trail back can be tedious. I have been looking at this history trying to trace the historical changes in the technical side of theater in the US.
New Mexico State University has a variety of PDF's available about doing research and using the library. I know that all of the schools that I have attended have the same. Connection to a college or university is nice because you often will have access to digital copies of articles that you may not otherwise have access too. Many articles will not be available electronically though, so you still have to use the actual library!
Internet Scout Project seams to be an online content knowledge management system where users can create their own database, but also provides databases to the public. One such site is the Electro-Mechanical Library available which has a large collection of knowledge about devices. This particular source is not necessarily research in terms of academic writing, but very helpful in terms of looking up information on specific electrical and mechanical devices.
Obviously, there are many, many, many more sources out there, but these represent a few that I have been using.