I'd recommend messing with some 18650 Li-Ion cells (I highly recommend protected ones unless you like explosions), at 3.7v 2000mah, that's equivalent to a 6000mah 1.5v cell, but smaller and lighter. Two of them, and a switchmode DC-DC converter (don't use a linear regulator) should last for a fair while.

Batteries: http://www.dealextreme.com/details.d...979~r.45363994

Chargers: http://www.dealextreme.com/details.d...151~r.45363994, http://www.dealextreme.com/details.d...105~r.45363994 or http://www.dealextreme.com/details.d...499~r.45363994

Much smaller and lighter than a D cell.