** Preowned ** Official Microsoft 120Gb hard drive for the Xbox 360. Comes with 12 months warranty.
You can either order online to be delivered to your door or If you're lucky enough to have a CeX local to you then you can try and see if they've got some in stock.
If you're upgrading from an old 20Gb or 60Gb (and want to keep your save games etc) then you can phone MS and ask them for a free transfer cable. It will take around 1 to 2 weeks to arrive.