If you’re interested in early publication, some light Internet searching will likely turn up the title you need. In the interest of going straight to the source, I like to start with an author’s official webpage. There’s usually a list of publications. If the publication dates aren’t included, do a quick search at a website that sells books. The publication date should be listed in the product details.
If you’re interested in looking at work that hasn’t been published or collected into a book by the individual author, the search becomes trickier (and perhaps more of an adventure.) You might do some investigation to find if the writers’ archived materials are housed in a library. For example, the Harry Ransom Center at the University of Texas at Austin houses the archive of author David Foster Wallace. The archive includes handwritten early draft pages of his novel Infinite Jest and “Viking Poem,” which he wrote when he was a child.
You might also look into publications that have explored this territory. First Fiction: An Anthology of the First Published Stories by Famous Writers, edited by Kathy Kiernan and Michael M. Moore, includes early professional publications from a variety of authors, including F. Scott Fitzgerald, Joyce Carol Oates, Margaret Atwood and Kurt Vonnegut. If you’re interested in going back even further and looking at juvenilia, work created in the author’s youth, you might look at First Words: Earliest Writing from Favorite Contemporary Authors, edited by Paul Mandelbaum. This collection includes fiction by Amy Tan, Stephen King, John Updike, and Rita Dove. Writing from childhood offers up something very different than early professional publications, but each can give a revealing glimpse into a specific aspect of the creative process.