I believe you will only be able to get 2.5" usb drives that dont need a psu.
you could get these:
http://www.overclockers.co.uk/showproduct.php?prodid=HD-032-BT
http://www.overclockers.co.uk/showproduct.php?prodid=HD-015-FJ
Save yourself £18 and the caddy looks pretty.