The official subject heading for letters is "correspondence" though sometimes it is listed as correspondences. I suggest using correspondence* as a subject. Combined this with an individual or topic.
Since we are looking for unpublished materials, you can narrow the publication date on this one, either in the WM catalog or in WorldCat
You can also search for a name or topic, and search Letters within the Title field. Again, limit publication dates.
When searching in the catalog and worldcat, use the Archives tabs to narrow specifically to archival materials.
This will include individual items and whole collections.
Some highlights