Cheapest I could find with free postage.
The Wireless Network Adapter for the Xbox 360 has been designed with versatility in mind. The adapter seamlessly synchronises with the Xbox 360 system to enhance your gaming experience.
The Wireless Network Adaptor is compatible with Windows Vista Media Centre, and allows you to stream photos, movies, music and videos wirelessly from your computer, and play games using Xbox Live.
Powered by your Xbox 360, the Wireless Network Adaptor reduces cable clutter and is dual band-compatible (5 GHz and 2.4 GHz).