The traditional starting place for any librarian has been the Statistical Abstract of the United States. It is useful because it brings together a wide variety of statistical information from many different government agencies. You can look up a topic and then use the table information to track which agency may have more data. It is also an excellent resource for historical statistics and you can access earlier editiions online.
Because Congress discontinued funding for the compendium, ProQuest has created a subscription database and a print version starting in 2013. You can contact ProQuest for more information about the PQ Statistical Abstract.
In addition to the Census Bureau and census-related resources, there is strong tradition of statistical programs within the United States federal government. A comprehensive list of federal agencies with statistical programs can be found through FedStats.