Where can I buy an xbox 360 Slim power cable?

I have a feeling my power "brick" is faulty and need to find a replacement, hopwever, none of the high street retailers "Game, Gamestation, HMV, Argos" seem to stock them. I've looked on E-Bay and play.com etc but can only find power cables for the old-style 360 models (The one with the square style adapter) I need one for the new style 250gb "Slim"xbox 360