Take cues from the literature you already have for possible data sources. Early on, search the following resources, which will either lead you directly to data or will give you vital information about who collects the type of data you need.

At the very first stages of your literature review, start taking notes on potential data sources.  Make a habit of jotting down the data used in each study you read to make it faster when you come back later in your search for data.  Also, this practice can help you see and articulate how your contribution is unique.  You might want to keep these notes in a table like the following for easy reference.

For an editable table like the one above, open this spreadsheet, save a copy for yourself, then use it to track your literature.

See also