MLA
Waller, William. Letter from William Waller to "My Dear Sir," Regarding the Purchase of Bonds - July 16, 1867. 1867. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 9, Folder 42. https://digital.library.emory.edu/purl/040xpnvz2k-cor.
APA, 6th edition
Waller, W. (1867). Letter from William Waller to "My dear sir," regarding the purchase of bonds - July 16, 1867 (Box 9, Folder 42) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/040xpnvz2k-cor.
Chicago
Waller, William. “Letter from William Waller to ‘My Dear Sir," Regarding the Purchase of Bonds - July 16, 1867.” Letters (correspondence). Civil War Collection, 1867. Box 9, Folder 42. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/040xpnvz2k-cor.