The U.S. Census Bureau (part of the Department of Commerce) published a much-reprinted booklet, Bureau of the Census: Fact Finder for the Nation (1948). Although many other government agencies and bureaus collect facts (the Labor Department, for example, records labor statistics), only the Census Bureau is known as the “Fact Finder of the Nation.”
 
The moniker “Fact Finder of the Nation” has not been trademarked.
   
 
Wikipedia: United States Census Bureau
The United States Census Bureau (officially the Bureau of the Census, as defined in Title 13 U.S.C. § 11) is the government agency that is responsible for the United States Census. It also gathers other national demographic and economic data. As part of the United States Department of Commerce, the Census Bureau serves as a leading source of data about America’s people and economy.

The most visible role of the Census Bureau is to perform the official decennial (every 10 years) count of people living in the U.S. The most important result is the reallocation of the number of seats each state is allowed in the House of Representatives, but the results also affect a range of government programs received by each state. The agency director is a political appointee selected by the President of the United States.
 
OCLC WorldCat record
Bureau of the Census: fact finder for the nation.
Author: United States. Bureau of the Census.
Publisher: pp. vi. 50. Washington, 1948.
Edition/Format:  Book : Government publication : English
 
Google Books
We Count in 1950:
A U.S. census handbook for elementary school teachers
By Frank W. Hubbard; United States. Bureau of the Census.
Washington, DC: U.S. Dept of Commerce, Bureau of the Census
1950
Pg. 4:
A FACT FINDER FOR THE NATION
The source of many of our social and economic statistics is the Bureau of the Census. We do not always know this.

Google Books
An Introduction to Business Management
By Harold H. Maynard, Walter Crothers Weidler, et al.
New York, NY: Ronald Press Co.
1951
Pg. 121:
United States Bureau of the Census.—An appropriate description of the Bureau of the Census is “fact finder for the nation.” The statistical materials derived from the various censuses and surveys of the bureau have wide application to all phases of business activity.
 
OCLC WorldCat record
Bureau of the census, fact finder for the Nation.
Author: United States. Bureau of the Census.
Publisher: Washington, 1957 [i. e. 1958]
Edition/Format:  Book : National government publication : English
     
21 December 1958, New Orleans (LA) Times-Picayune, “Current Census in N. O. One in Many for Bureau,” sec. 3, pg. 9, col. 2:
The current “special” census of population just conducted in New Orleans, although of importace to the city, was but a small operation in the vast working schedule of the Bureau of the Census—“The Fact Finder for the Nation.”
 
OCLC WorldCat record
Bureau of the Census—fact finder for the Nation.
Author: United States. Bureau of the Census. Statistical Information Division.; United States. Bureau of the Census.
Publisher: Washington, U.S. Bureau of the Census; For sale by the Supt. of Docs., U.S. Govt. Print. Off., 1970.
Edition/Format:  Book : National government publication : English