Some of the data sources listed in this guide are freely available, but many are resources purchased/subscribed to by Princeton University Library. These are useful starting points, but are by no means all that the Library provides access to. Contact me if you need help.
The Librarians at Carleton University have a great guide on a systematic process for locating and evaluating research data. It encapsulates the questions a research librarian will help you think through in approaching your topic.
In addition to the data sources listed to the left, be sure to look at the following data repositories: