Consult the Rutgers Libraries Libguides, particularly those owned by me, Ryan Womack, Data Librarian
- Census Data Portal portal to US Census information (major source of demographics)
- Sage Data compilation of official summary statistics
- Statista quick lookup of current facts
- Social Explorer easy to browse, map-based guide to Census and other demographic data
- Data.gov massive listing of open datasets from government sources, often in very raw form
- ICPSR largest archive of social science datasets
This is only a tiny selection. Most data questions are very individualized and depend on finding the right producer of the data.
You can contact me for further assistance using the email and Appointment buttons on my libguides profiles.