(This is abbreviated text from The Global Indie Author.)
If you elect to use the ISBN system, and to purchase your own numbers, you do this before you put your book on the market as you must include the number on your copyright page.
In some countries the ISBN agency and national registry are one and the same—in Canada, they are combined into Library and Archives Canada—while in others they are distinct agencies: in the U.S., Bowker is the ISBN agency while the Library of Congress is the national literary registry. You thus need to determine who administers and/or sells the ISBNs for your country.
United States and its Territories
In the U.S. and its territories, Bowker sells a single ISBN for $125.00; a 10-block of ISBNs costs $250.00; a 100-block of ISBNs costs $575.00; and a 1000-block of ISBNs costs $1,000.00.
Canada, New Zealand, and South Africa
In Canada, ISBNs are free to publishers including self-publishers. You must register first with Library and Archives Canada and be vetted, after which you will be given an account; the process takes about 10 to 14 days. ISBNs are also free in New Zealand through the National Library of New Zealand, and in South Africa through the National Library of South Africa. These latter registries may also have a vetting process because of the free service, though I have been told anecdotally that New Zealand does not.
UK and Ireland
[UPDATED 6 August 2019 at the request of the UK and Ireland Nielsen agency; thank you for the information.]
In the UK and Ireland, purchases are made through the Nielsen UK ISBN Agency. ISBNs are sold in a minimum block of 1 number at a cost of £89.00, and £164.00 for a block of 10; both prices are inclusive of VAT. A publisher can elect to buy larger blocks at a progressively diminishing rate: 100 numbers costs £369.00, and 1000 numbers costs £949.00.
In Australia, the ISBN agency is Thorpe-Bowker, affiliated with Bowker in the U.S. In Australia, new publishers pay a one-time registration fee of $55.00. ISBNs are then purchased individually at $40.00 or in a block of 10 ISBNs for $80.00, a block of 100 for $435.00, and a block of 1000 ISBNs for $2,750.00.
How Big a Block Do You Need?
When considering how big a block to buy, remember that each format of your book will require its own ISBN, so if you release a single title as a hardcover, paperback, Kindle, ePub, and PDF, that is five already. ISBNs do not expire, so while you may have to pay in advance for the whole block of ten, they sit in your account until you need them. Similarly, when applying to an agency for free ISBNs, you will be asked how many publications you are likely to publish in the next year or how many ISBNs you will likely need; this determines how big a block they give you.
Registering for an ISBN Account
When you register as a publisher in order to acquire ISBNs, you may do so under your name, under the name of a company you control if the rights to publish your book have been assigned to it, or under an imprint regardless of whether the imprint is formally registered or not. You cannot open an account under a pen name or any other pseudonym.
In all the registries I checked, registration and purchases can be made online. It is generally a straightforward process: you fill out your personal information and pay by credit card if applicable. A few things to note, though: if you decide to create your own imprint or register under the name of a company you own or control, pay attention to whether the registry wants your personal name as the account holder and your imprint listed as the publisher, or whether they want your imprint/publisher name as the account holder and your personal name as the account contact and administrator. Each registry is a little bit different.
When you assign an ISBN to a title you do not need to input all information at once, or even ever. In fact, if you are publishing a print version you have to assign the ISBN before you can produce the book and fill in the information on, for example, the book’s dimensions: how can you know what the size will be until it is made? It can be a bit confusing at first but mandatory fields are usually clearly marked so you will know what must be inputted up front and what can be added later. So make a note of what is mandatory for your registry when you set up your account so you will have this information ready when you set up your title. At minimum you will need your title and names of all contributors, the book format, publisher or imprint, price, and geographical territories in which the title can be distributed. Later on you can add a product description (your book’s marketing synopsis), your primary Book Industry Standards and Communications (BISAC) categories, a cover image, contributor’s bio, product dimensions, et cetera. Obviously, the more info you provide to your potential buyers the better.