ISBNs are free for citizens and are issued by Library and Archives Canada. United Kingdom: The official agency is Nielsen BookData.
Because you need a (e.g., one for paperback, one for hardcover, and one for EPUB), buying in bulk is often more cost-effective. Buy ISBNs | Bowker | Identifier Services how to buy an isbn
The sole official source is Bowker, which operates through its site, MyIdentifiers.com. ISBNs are free for citizens and are issued