In the Library main catalog, use the term Letters as a KEYWORD or TITLE term in one search box, and your topic as a keyword in the second search box.
Then limit the search results to your time period of interest. After you get your results, limit the results to archival materials & manuscripts
This trick also works in WorldCat:
Search Letters as a Subject or Title term, Then search your topic as a keyword, then limit to your time period.
Under advanced options, limit to archival materials.
NOTE these materials typically do not circulate and cannot be ordered via ILL because they are held in a university archive. You can, however, go to the archival website and see if the material has been digitized.
NOTE some of these are published letters too, so pay attention to the source.
In the search results, limit to "letters" as a source type.