The manuscript reading room at the Library of Congress houses an extensive collection of Niebuhr's papers, including correspondence, typescripts, and photographs. This is the best place to start for researchers of Niebuhr's life. And, Supreme Court Justice Felix Frankfurter's collection is held in the same location and contains over 100 letters involving Niebuhr.
Union Theological Seminary and Presbyterian School of Christian Education in Richmond, Virginia holds an extensive collection of recordings by Reinhold Niebuhr on CD that are available for purchase. Most of the audio was recorded after Niebuhr's stroke in 1952.
The Columbia University Oral History Research Collection contains a 1953 interview with Reinhold Niebuhr. The interview includes his comments on his early life and education, his experience in Detroit, and his theological reflections of the time. The collection holds a transcript of the interview; the audio was not preserved.
This site contains free electronic copies of Niebuhr's writings, including The Irony and Destiny of Man, Moral Man and Immoral Society, and the essay "A View from the Sidelines." It also provides a selection of work by Reinhold's brother, H. Richard Niebuhr.
Sponsored by the United Church of Christ, the site features a number of letters from 1932 exchanged between the two brothers, as published in Christian Century, about whether U.S. military intervention in the Japan/China conflict of the time would be a "just" or "unjust" war.
The King Papers Project is a major research effort to assemble and disseminate historical information concerning Martin Luther King, Jr. and the social movements in which he participated. The site includes King's early 1950s essays "Reinhold Niebuhr's Ethical Dualism" and "The Theology of Reinhold Niebuhr."