I lucked onto a few rechargable L-ion battery packs for JVC video cameras by Energizer, for a buck each at Goodwill. Took each one apart and found 4 nice Panasonic lithium ion cells, each about the size of a AA and with tabs attached. The cells are a lower capacity than the OEM Apple cells - but the smaller size meant I didn't have to worry about cramming my new hand-wiring into the tight battery shell. I get about 3 hours on a full charge, not bad for a $3 investment in materials.