You can buy it from Amazon (if you're in the UK) or from Wordery or Book Depository (both of which come with free worldwide shipping). It's currently 25% off at Book Depository, so that seems like a good option.
Alternatively, if you are in the Milton Keynes area tomorrow, feel free to stop by the Waterstones and get a signed copy. I'll be there from noon and I'd love to see you.
If you can't stretch to a physical copy, the ebook is still 99p (UK) or $1.99 (US), but I'm told that's about to end. So grab it now!