Barnes & Noble can provide you with a FREE ISBN for your books listed on their site.
An excerpt from their site:
Can B&N Press provide me with an ISBN?
Answer: B&N Press can provide you with a free ISBN to publish your print book. We'll also automatically register your ISBN information with BooksInPrint.com®. This a global database of print books maintained by Bowker (the U.S. ISBN agency) and consulted by publishers, libraries, and booksellers. The "Publisher" field on your paperback's B&N Press detail page will read "Barnes & Noble Press." This will not appear in your interior and is only provided on your book’s detail page. This free ISBN can only be used on B&N Press for distribution to Barnes & Noble and BN.com. It cannot be used with another publisher or self-publishing service.
Click here to read more about ISBN's on the B&N website