I take it that you mean G2 /ubbthreads/images/graemlins/smile.gif
You can use the surefire kit (http://www.surefire.com/maxexp/main/co_disp/displ/prrfnbr/212/sesent/00) but that makes it way longer.
But you can also use pila's (http://jsburlys.com/).
A pila 168S battery will fit, but you also need the 3.7v lamp and offcourse a charger, which can all be found on ^^ the site of J.S. Burly's.
I don't know if using pilla's is the cheapest way, but i think it's the best way.