Several U.S. government agencies provide one-stop-shops for GIS data and, consequenty, are usually the best place to look first for existing GIS data. Listed here are some of the biggest and richest collections.
Finding the right data for your GIS project can be challenging. Before you start your search, be sure you can articulate what kind of data you need. Selecting the right data in the beginning will save time in the end.
If you cannot find what you need, contact a librarian to help. Your liaison librarian will know about data sources in your discipline, and Kristin Partlo knows about GIS data sources and uses generally.