There is only one place I know where you can read City of Ashes on-line. At the moment on Amazon, it is allowing kindle subscribers to read the book for free. Although subscribing to kindle will set you back £7.99 a month, it does allow you to read many other books on-line for free.
If you do not wish to subscribe then you will have to buy the book. I have read this book many times and is one of my favourites, so in my opinion it is definitely worth investing in!
Amazon is usually the first place I check when I am looking to buy a book, and you can find City of Ashes here on kindle for only £3.66.
They are also selling second hand paperback copies for only 0.01p! Amazon also now have the option of buying an audio version of the book.
Any major bookshop will also sell the book but this may be a more expensive way of doing it, although you won't have to worry about any postage costs.
The cheapest way that you could get your hand on it may be to check out your local charity shops. The books are usually set out in alphabetical order so it would be easy to pop in every now and again to see if they have got a copy. You may be able to find one for even less than a pound!