So far Microsoft have only been offering their 250GB Xbox 360 hard-drive as a standalone accessory in Japan; everywhere else it's been included as part of a bundle with the console itself. That looks like it could be changing, however; one 360 gamer spotted the standalone drive being advertised on Microsoft's own Xbox Dashboard.
So far there's no sign of the 250GB hard-drive on Microsoft's official accessories webstore, despite what the Dashboard update might suggest, but we're guessing it'll make an appearance sooner rather than later. In Japan it retails for the equivalent of around $170; in contrast the 120GB drive is currently selling in the US for $130.
Update: Microsoft have confirmed that the drive should go on sale today, priced at $129.99. It will be bundled with the data transfer cable.