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.
This compendium of statistics produced by the U.S. government contains basic tables summarizing the vast amount of data collected by the government. Find a table here that is related to your topic and follow the cited source to identify which agency collects the data you need. Also available in print in the Ready Reference collection at C 3.134: 2012.
Search summaries of statistical publications, then link to the full-text of selected publications on Statistical Universe and government Web sites. This comprehensive database includes the three major statistical indexes (American Statistics Index, Statistical Reference Index, and Index to International Statistics).