In addition to the Major Research Collections, the Bishop Collection, and Dartmouth’s holdings (see Significant Collections), all of which hold the major reserves of manuscript material, the following institutions hold an appreciable amount of correspondence related to Mosher:
- Haverford College Library. The Morley family papers in the Quaker Collection include correspondence from Mosher to Christopher Morley. Mount Holyoke College Library.
- The Henry E. Huntington Library and Art Gallery. According to The National Union Catalogue of Manuscript Collections, there are 174 letters covering the period 1894-1924 from Thomas Bird Mosher and others relating to business matters and the publication of the Mosher books. This was a gift from W. W. Clary in 1948.
- Library of Congress. The papers of Horace Traubel and his wife Anne Montgomerie Traubel include correspondence to and from Mosher.
- Princeton University. Among the Arthur Symons Papers there are 19 letters from Arthur Symons to Thomas B. Mosher, a letter from Rhoda Symons to Mosher, and a letter from Mosher to John Quinn.
In addition to the institutional collections listed above, there are manuscript holdings at UCLA, Haverford College, University of Virginia, Temple University, University of Iowa, Cornell University, University of Texas at Houston, Bryn Mawr College, University of Michigan-Ann Arbor, Columbia University, New York Public Library, Smith College, Wellesley College, Newberry Library, and at the British Library.