How does a curator find work for an exhibition? How does he know that the right works are in a particular private collection or museum?
Many works are not printed in catalogues, they are in storerooms, and if this is a private collection, then you may not even know about the location of a particular work. For example, a curator creates a large monographic exhibition of an artist and wants to collect as many little-known works from private collections as possible: how does he act in this case?
In fact, professionals know a little more than is known in the public field) Although now, in the Internet age, a lot of things can be found out simply by using a search, including such information.
For example, there are reason directories. This is not just an exhibition catalog or a book about an artist. The reason catalog is usually an exhaustive database of works by a particular author. This catalog is prepared by specialists on a particular artist for several years, if not dozens! In such a catalog, absolutely all works attributed by specialists to the brush of a particular master are mentioned, sizes are indicated, participation in exhibitions, all possible mentions, the full history (who bought/gave/sold and when). Accessing such a directory can tell you where to look for a particular job.
If we are talking about an artist for whom there is no such catalog, then you can contact either him, if he is alive, or with his heirs/foundation/heritage managers. You can go to the artist's website (fortunately, many artists have them), or to the museum where his works are stored – usually local specialists understand where other objects can be found. The names of the biggest collectors and biggest sales are also well-known in the art community.
Sometimes, of course, the search for works is similar to a true detective story, when information is collected bit by bit about sales that happened centuries ago, there are some references in decayed museum documents, etc. But often everything is a little easier than it may seem from the outside)