I would strongly encourage you to support your local bookstore. If you want to know more about why, then have a read of this which raises some of the issues involved. Here’s a good access point to find your local store.
You can of course choose to buy online here:
If you want it electronically, the Kindle version is available from Amazon.
The audio book is also available in the same places.
Buy in any good bookstore, or online here:
Otherwise, you can grab the Kindle edition at Amazon.co.uk
Buy in any good bookstore, or online here::
Buy direct from the publisher in the Netherlands here.