A Finding Aid is basically a guide which helps you navigate through an Archival Collection. It is comprised of many elements. However, there are four main elements.
An example of a finding aid for a collection held at the Adelphi Special Collections is the William Hone Manuscript Collection
You will have to provide the title of the collection and when possible the number of the box and folder.
Finding Other Archives' Collections:
There are databases to help you find other archival repositories and collections that might match your research question. To start, try ArchiveGrid and you can also try WorldCat and Google.
Talk with an archivist! Archivists have knowledge about the landscape of institutional collecting and can point you in a good direction.
Finally, you’ll want to interpret your findings. Ask yourself what is the context in which these manuscripts etc. were created? What are the relationships within the collection or perhaps among several collections? How can I use these primary sources, photographs, manuscripts etc. in my scholarly work and or perhaps in exhibits? Be mindful of copyright regulations. Where repositories may have authority to allow use of some materials and some reproductions, this is not true for all materials found in the archives. There are also rules for fair use.