Want to get into the library distribution systems with your self-published works?
IngramSpark is the place for you.
Expect an error message from IngramSpark to the effect that your ISBN is already in use (because KDP has registered your ISBN with Ingram).
How do you fix that?
Complete a Title Transfer Addendum at the bottom of this page and email it to the address noted here. Lightning Source (IngramSpark) will email it to KDP, who will in turn email you to request confirmation.
In my experience, it took about three days.
Here’s an excellent article on why to use both services by Karen Myers over on the ALLi website.