I've got one of these, seems to work OK but as Dave says, you need phone a phone signal.
http://www.ebay.co.uk/itm/Mini-Real-Time-Magnetic-GPS-Car-Vehicle-Tracker-Personal-Tracking-Device-TK102B-/111201679112?pt=UK_In_Car_Technology&hash=item19e422f708
The one I got had a different logo on...