Although you can still find those gigantic hard-to-navigate "Marketplace" books in the reference section of Barnes and Noble, you really don’t need them. I believe the best place to find a literary agent is on your bookshelf.
If you’re serious about writing, your bookshelf—whether physical or digital— is already well-populated. If you don’t have many physical books on the shelf, think back to books you’ve checked out from the library in the last couple of years, or do a deep dive into your e-reader for the books you’ve enjoyed.
Here's how to put your bookshelf to work for you:
Look at recently published books that you admire, books published in the genre in which you are writing. Is your novel literary coming-of-age? Is it a thriller? A romance or cozy mystery? Sending a query to an agent who doesn’t represent books in your genre is pointless, so identify your novel’s genre before you search for agents.