Wherever you are in the world there is just one option to get an ISBN without any problems coming along with it. The International ISBN Agency appoints just one official agency within each of its national/regional jurisdictions from which to purchase ISBNs. For instance, if you are in the United States, the ISBN agency is R R Bowker of New Providence, New Jersey, and in the United Kingdom as well as The Republic of Ireland the ISBN agency is Nielsen Book Services in London. For contact details of the USA and UK ISBN agencies as well as all of the others then you will find a big list on our ISBN Agency page.
If you are determined to find a cheap way to get an ISBN then there are some things to consider. The first thing to know is that it is the publisher of the book that holds the ISBN. If you manage to find someone other than an official ISBN agency for your jurisdiction that will let you have an ISBN, then you will not be simply buying an ISBN, but handing over publication to someone else - it is they and not you that will be the publisher of the work. In that case, you need to ask yourself what exactly you are handing over along with the cash for the ISBN.
Having said that, then it must be said that there are some very trustworthy and reliable organisations that will publish a book on your behalf, but you must be aware that there are some sharks, too. You need to very careful and find out all you can about self-publishing before you hand yourself over to someone else that will provide you with any publishing services. Don't enter into any contracts and don't hand over any money without being sure of what you are getting yourself into. You really should take business advice from someone that knows all of the pitfalls.
Be aware that unless you have purchased the ISBN yourself from your region's official ISBN agency then it is not you that will be the publisher of your work. But perhaps that is what you want. You are happy to have some other company publish your book for you. That's great as long as that company is one of the many reputable publishers. Many of them are household names and many more are not so well known. Getting published that way never entails you handing over any money. If you are one of the lucky (or talented) few that has found a publisher that thinks you have written a book that will sell then it may even be the other way around - they will be keen to give you some money in advance of publication. Very best wishes to you!
Don't forget that self-publishing is now the option preferred by many successful authors. If you like being in total control, then this may be the option for you. See Kindle Direct Publishing for one very popular option.
For contact details of the USA and UK ISBN agencies as well as all of the others there's a big list on our ISBN Agency page.