The MLA Handbook identifies nine core elements for entries in a works cited list. Some of these elements will be infrequently used for citing archival collections, while others will almost always be used. As you build your citations, keep the following in mind:
Author: Archival sources often do not have an obvious author or creator. If you do not know the author or creator, MLA style directs you to skip the author element and begin the entry with the source's title (see MLA Handbook, page 24.)
Title of source: Many archival sources are untitled. For untitled sources MLA style directs you to provide a generic description that is not italicized or enclosed in quotation marks in place of a title (see MLA Handbook, pages. 28-29.)
Title of container: Archival collections can be considered "containers" in MLA style, because a single item in an archival collection is part of a "larger whole," the collection itself (see MLA Handbook, pages 30-36.)
Item Location: You may want to include specific location information for archival materials if your reader will need it to find the original source. This could include the numbers for the box and folder in which you found the item (see MLA Handbook, pages 51-53.)
Collection Location: Archival collections are unique, and are almost always located in a single institution. As such, rather than using a page number as you would for a book or article, you should use the name of the institution and its city as part of the location in your citation (see MLA Handbook, pages 49-50.)
MLA provides a quick guide and practice template to help construct citations. If you're confused about how to cite a particular item or collection you can fill the template out and go from there.
The citation would look like this in your works cited list:
Sell, John M. Letter to William Sell. John M. Sell Civil War Collection, 3 November 1861. Box 1, Folder 3, Special Collections and Archives, Universtiy Library, California State University, Northridge, Los Angeles.