Finding information on the Web is easy. Find the right information can take more time. A Google search on the term "music history" for example will give you some useful sites, but few that may be of use to your academic needs. If you are getting to many results that are not what you need, try doing an advanced search limiting to .edu or .org sites.
Follow the link and fill in your keyword(s) in the top box, such as "costume" for example.
The Online Resources for Music Scholars, the first web page listed below is a large and comprehensive guide to websites suitable for music scholarship. It is probably the best place to start looking for reliable websites related to music history topics. The other sites listed below it are just some samples of the many available on the web. Be cautious of web-based information. Information from a scholarly organization or educational institution is more likely to be reliable than information from a random individual's web page.