If you are writing the paper for a university level course, I would suggest starting with your library's resources. Most universities provide their students with online access to a variety of reference databases, such as Cambridge Histories Online. These are good places to get a brief overview of the topic or see who is writing about what (Cambridge has several great medieval history volumes). Journal databases like JSTOR tend to produce specific articles; while narrowly focused, they can supplement your research nicely. JSTOR also has a sizable collection of book reviews that can come in handy when evaluating secondary sources. When reading through scholarly databases or articles, pay attention to the footnotes and endnotes. Which primary and secondary sources are they citing? This will help you find more sources to add to your research list.

Of course, there is nothing like spinning through your own library's catalog for any books they might have on the subject. If you notice a source (primary or secondary) that comes up a lot in your readings, but is not listed at your library, World Cat can help you track down the nearest available copy. Most university libraries are good about inter-library loans.

Specific to Medieval Studies, you might also want to check out Fordham's Internet Medieval Sourcebook, a primary source database that has save by butt on more than one occasion.

/u/AnacreonInHeaven had a fantastic write-up about a month ago on a similar topic concerned with how to separate good sources from the bad. You may find their comments and suggestions helpful as well.