3-LED Cree 3-Mode Regulator + Reflector 2x18650 Module Set (3.6V-9V) $28.27
http://www.dealextreme.com/details.dx/sku.9358

You'll need to fabricate a cap with switch for the back where it would screw on a Cree flashlight battery case.

Unsolder the springs and replace with wire to an external battery pack.