There exist a variety of union lists and shared catalogue for archival and manuscript (AM) material in the world. There is some overlap in coverage and holdings, but to find everything, one must search each of these. Some deal with specific jurisdictions (indicated by the flags on each item), and some deal with wider geographic areas.
It can sometimes be difficult to identify an author's "main" collection - there may be manuscripts and other material in more than one archive, and that author may be represented in a vareity of other collections or archives (their correspondence, for example, with other authors, may be in that other author's collection).
Most of these tools use Library of Congress Subject Headings (LCCH), which is what academic library book catalogues use to describe their holdings, but there are also a variety of archival description standards that are in use across the English speaking world:
Maintained by the International Council of Archives :
Some biography sources will point you to an author's archival and manuscript collections: