Use Biographical Directory of the United States Congress, Library of Congress, to find out where a member's papers are held. From a member's entry, choose "Research Collections".
Congressional Collections, National Archives and Records Administration, The Center for Legislative Archives.
A list of "archival repositories and congressional collections."
George J. Mitchell papers, George J. Mitchell Department of Special Collections & Archives, Bowdoin College Library.
"The collection documents Mitchell's professional and political career, including his work in Senator Muskie's 1972 presidential campaign, Mitchell's own 1974 gubernatorial campaign, his career as senator from Maine, his six years of service to the nation as Senate majority leader, his work in Northern Ireland, and his career and activities since 1998."
American Congress Digital Archives Portal, West Virginia University Libraries.
"Congressional archives are the primary materials generated by Congress and that document the work of Congress. They are separated into official records and personal papers. Personal papers, which this portal features, include materials created or received by an individual Member of Congress as documentation of their career, the organization and functions of the office, and the work of office staff."