Every book requires an International Standard Book Number (ISBN). In the U.S., the ISBNs are purchased from Bowker Identifier Services. As the U.S. ISBN Agency, Bowker is the ONLY official source of ISBNs in the United States.
What shoudl be noted for all first-time self-publishers is that every version of your book: eBook, softcover, and hardcover needs its own ISBN. Furthermore, if you update the book, essentially creating a new version of the book, it requires a new ISBN. Conveniently, you can purchase groups of ISBNs from Bowker.