If you’re looking to map people across the US, there is no source of information with as much comprehensive coverage at the Census data. Of course it might not be the type of information you want, but you’ll have to take that up with Congress.
Meanwhile, here are a couple of sources for it:
American Fact Finder, info from the source itself. Be brave and dive in.
Social Explorer, a long-time favorite, especially if you just want to LOOK at Census maps.
the National Historical Geographic Information System. A funny name for what it is: a good source for raw historical data. Plus the only source I know of for historical boundary files.
