In JabRef, choose BibTex -- New entry ....
In the lower part of the pop-up window, there is the box "ID-based entry generator"
In the field ID type, select ISBN.
Enter the ISBN of the book, and click on Generate.
There are a few online BibTEx entry generators for books. Always check the entries to make sure all the BibTex fields (e.g., author, title, publisher) are accurate.
ISBN to BibTex Converter