There is bound to be something in at least one of these databases that will help you locate some pertinent primary documents for your paper.
Atlanta Constitution (1868-1984)
Atlanta Daily World (1931-2003)
The Chicago Defender (1909-1975)
Chicago Tribune (1847-1993)
Christian Science Monitor (1908-2003)
Detroit Free Press (1831-1999)
Los Angeles Sentinel (1934-2005)
Los Angeles Times (1881-1992)
The Michigan Chronicle (1939-2010)
The Nashville Tennessean (1812-1922)
New York Amsterdam News (1922-1993)
New York Times w/ Index (1851-2013)
Pittsburgh Courier (1911-2002)
The Wall Street Journal (1889-1999)
The Washington Post (1877-2000)
You might find some pertinent primary documents in these books.
The Library of Congress's American Memory site has some great sports collections, especially of baseball. Here is one you might find useful.