In the US, RadioShack has those AA cases. I use one as a battery extender/emergency charger for N770/N800 and other devices (iPod, etc) which can charge from a USB port. For the N770 & 800, you simply solder a nokia tip to half of a standard USB cable, and you're good to go. I get about 20+ hours or about 3-4 full recharges out of a set of NiMH batteries.

By the way, I saw something similar at Wal-Mart, for about $10. It's just a pack for 4AA's with a USB socket on it. But, would any of us be buying Internet Tablets if we didn't like to tinker ourselves!?