If you publish through Amazon exclusively, they'll give you a number of their own. But it's theirs, not yours, and you can't sell on any other platform with that number.
Most aggregators (companies who help with formatting and distributing your book to sales platforms) can assign an ISBN as part of their fee, and while technically you've paid for it, I don't like the idea of tying my work to a number purchased by someone else. Companies can go under, and it's a worry that's not worth adding to the pile of my day-to-day worries.
Every country has its own rules and pricing as far as ISBNS are concerned. In Canada, for instance, ISBNs are free. In the good old USA, not so much.
One ISBN is $125. Being that they're only available from one place, there's no competition in pricing.
Ten ISBNs, however, are $295. So an abrupt drop from $125 per to $29.50 per. The problem quickly becomes Songbird/3 ISBNs, A Wider World/3 ISBNs, and Lady, in Waiting/3 ISBNs, and that ten is almost gone. What if I want to do a hardcover edition? Or a large print, which would be nice for library purchase? Or a box set?
Enter the jumbo pack - 100 ISBNs for $575. Or $5.70 per number. Can I tell you how much I hate having to overpurchase to get a deal? But if I didn't the hundred, I'd be paying even more for twenty, which makes absolutely no sense at all, except that the ISBN people make money hand over fist and we writers panic at the vast expanse of numbers ahead of us, wondering if we'll ever be able to use them all before we're too old to remember how to write.
And no, before you ask, they're not transferable. I checked. They can be purchased by a group of writers who hold themselves out as a publisher, but that means you have to share a press name (and potentially house style, and absolutely tax consequences) with other people, and I've just gotten everything back tog act independently, so that seems counter-intuitive.