Essentially, you can get it anywhere they sell books. Here are some examples:
If you have a local independent bookshop, please support them and get it there – even if that means you have to ask them to order a copy for you.
A great second-best choice is Hive. They supply books online, but also support independent bookshops
Waterstones have it too. I’m generally in favour of Waterstones. You can order it directly from them here.
If you like Blackwells (and why wouldn’t you?) they have copies.
And of course you can get it from Amazon – who also supply it in ebook form.