By now, this is a scam or at the very least a monopoly by Bowker. You can only buy ISBNs from them. And they are highway robbers. I think they're free in Canada (they used to be free here, I knew someone who had a hundred of them) and Bowker will charge you about $125 for one here.
What's an ISBN? From Wikipedia
The International Standard Book Number (ISBN) is a unique numeric commercial book identifier based upon the 9-digit Standard Book Numbering (SBN) code created by Gordon Foster, Emeritus Professor of Statistics at Trinity College, Dublin, for the booksellers and stationers W.H. Smith and others in 1966.
But Amazon and BN have their own identification system. They don't use ISBN for digital books.
It's true other sites do like iBooks. They require them.
I say the heck with you, iBooks. Everyone knows Amazon stocks the most books (most of everything) so everyone already goes there. The few who don't, so what. Self-publishers can't even get into iBooks, you have to go through another entity and often pay for the privilege of being listed there. Big whoop. I don't pay for anything I can do myself.
Q: Why do you need an ISBN?
A: AFAIC you don't
Q: I WANT one, I won't be happy until I have one. How do I get one?
A: The problem (monopoly) is that for every version, every edition of your book you need a different ISBN. If you want a paper book, you have 1 number. If you have a digital book, you have another. Ka-Ching! For Bowker. (A block of 10 will cost about $300, see how they do that?)
Q: You still haven't told me how to get one.
A: Go to Smashwords and publish your book through them and pay $10. Done. That ISBN goes with the book wherever you sell it. You can unpublish at Smashwords at that point if you want, who cares? If you want to print, buy it for $10 at CreateSpace. Done. This is not that complicated.
Q: That's it? Why does everyone make such a fuss?
A: No idea. But what you don't want to do is take the free ISBN offered to you at Smashwords or other such sites. They will be the publisher of your book until the end of time. If you don't object to that, do it.
Q: Do you object? You're the Queen of Free. The goddess of penny pinching.
A: And I am so proprietary. I publish my books. So, yeah, I object. $10 is the bargain price to pay for something I don't think you need.
Q: I'm so confused. Maybe you don't know what you're talking about.
A: I can confirm your initial concerns about yourself. You are confused.
Are we living beneath the volcano?
The great Jack Bruce