MTB batteries or a DX jobby.
Not had any experience of the MTB batteries ones but they they do get a lot of good press around these parts.
I’ve had a couple of the dealextreme ones and I would say avoid the £30 quid ones as the soldering is a bit piss poor and the beam pattern is too tight for off road. The more expensive ones like the one above are great for the money and I have never had a problem. Battery burn times are typically around 3+ hours. Trouble is if there is a warranty issue with them then the turn around is months not days.