I would get high capacity NiMH rechargeable batteries. That's what they recommend for digital cameras but I'm sure it would also apply in your case.
Locally I know Radio Scrap carries a kit that comes with a set of AA's and a recharger.
The best set up that I've heard about are the batteries and rechargers that you can get from a company called Maha. You need to order these from the States.
Here a NiMH rechargeable battery review I found.
http://www.imaging-resource.com/ACCS/BATTS/BATTS.HTM