The ISBN code or "International Standard Book Number" can be bought through various venues. If you are using a traditional publisher, they will most likely provide this service for you as part of the publishing service. If you are self publishing there are a few ways to go about obtaining the number. The first is to purchase an ISBN as part of the publishing package with the printer that you've chosen. For example, CreateSpace (owned by Amazon) provides a free ISBN, or allows you to choose custom ISBNs for nominal fees (very small fees). Your second choice is to purchase the ISBN from a reputable seller such as Bowker. Bowker sells ISBNs directly to the publisher (in this case, you). The best option is to purchase from a source such as Bowker, but to purchase a pack of ten ISBNs. One ISBN costs $125, whereas a pack of ten will cost $250. That is a huge savings if you know that you will be printing in multiple formats or have future editions (each requiring its own ISBN, mind you).
The ISBN of And Then There Was No One is 9780571238811.
Each country has an issuing authority - a web search should reveal who that is in your country. ISBN's are usually issued in blocks, rather than singularly, usually tens. Part of each one is a code for the publisher, which can be anyone (at least in the UK).
The ISBN (International Standard Book Number) is located near the bar code and on the reverse of the title page; it will be a series of numbers (how many numbers depends on the book's age) preceded by "ISBN". The ISBN was introduced in 1966, so books printed before then will not have one.
The ISBN of Room One is 9780689866869.
The ISBN of One of Ours is 0803214316.
The ISBN of One Christmas is 978-0394532660.
The ISBN of Moon Base One is 0571061338.
The ISBN of Let the Right One In is 0312355297.
The ISBN of One Month to Live is 9780739358498.
The ISBN of Economics in One Lesson is 0517548232.
The ISBN of DC One Million is 1563895250.
The ISBN of The One Minute Manager is 9780006367536.