You can pick them up on a popular auction site for around £55.
Or, you can bodge one yourself by buying a WD BEVS drive and obtaining a hard drive sector extracted from a 360 drive then running a utility to put the sector on the drive. This will cost you about £40, plus you have to sacrifice an existing 20GB drive to get the enclosure from. On top of that, using a 120GB drive's sector could lead to HDs being on Xbox Live with the same serial, therefore a potential ban risk. Hardly worth the effort to save £15 or so.