The confusion almost every new self-publisher hits at the same point: Amazon KDP gives you a free ASIN, not an ISBN, and Kindle-only sales don't strictly need one. But the moment you want to distribute wide — Apple Books, Kobo, libraries, other retailers — an ISBN becomes part of the picture, and that's where the free-versus-paid decision actually starts to matter.
Distributors like Draft2Digital, whose network reaches readers through Books2Read the way every book on this site does, will issue you a free ISBN for ebook distribution through their platform. It works. Retailers accept it, readers never see it, and for a large number of self-published authors it's genuinely the right call — it costs nothing and does the job.
The catch, and the reason to at least understand it before defaulting to free: the ISBN's registered publisher of record ends up being the distributor, not you. For most fiction readers this is invisible and irrelevant. It starts to matter if you ever want your own name or imprint listed as publisher in library and bookstore databases, if you're building toward a recognisable small press rather than a single book, or if you later move a title to a different distributor and want the ISBN to travel with it rather than staying tied to the platform that issued it.
Buying your own. In the UK, ISBNs come from Nielsen, sold in blocks — a single ISBN costs disproportionately more than buying a block of ten, so if you're planning more than one book, the block is almost always the better value. In the US, the equivalent is Bowker's MyIdentifiers service. Either way, once you own the block, you're the publisher of record for every book you assign one to, permanently, regardless of which distributor or retailer you route through later.
One detail that catches people out either way: each format of a book is technically treated as a separate edition, and needs its own ISBN — ebook, paperback and hardback each get their own number, even though it's the same book. Budget for that when you're costing out a block rather than assuming one ISBN covers every version.
My own practical answer, distributing wide across both markets: free distributor ISBNs are entirely fine for an ebook-only release, especially your first few books while you're finding your feet. But the moment paperback enters the picture — which for most self-publishers happens fast, since a wraparound cover and a POD listing are only a short step from an ebook already selling — I'd buy the block and own the publisher-of-record data outright from that point on. It's a small cost now against a genuine hassle later if you ever want to consolidate an author brand across a growing shelf of books.