If there's one locally, check prices at a model airplane hobby shop. They should have a wide selection of rechargeable batteries, both drop-in AA replacement and with solder tabs, and will know a good bit about them since they're now used as primary flight power for many radio control airplanes.
If you go with NiMH, you'll need a charger that's designed for that type cell, since overcharging them will destroy them in one go (NiCd are somewhat tolerant of mild overcharge); the charger must sense voltage, look for the "peak" and completely stop, rather than drop to trickle as most older NiCd chargers will.
The other battery type you might consider if you can get one that will fit your battery compartment is lithium ion/lithium polymer -- these have almost the energy density of alkaline primary cells (2-3 times NiMH, about 5 times NiCd), but are rechargeable and much lighter than NiCd or NiMH type as well. They cost a lot, but they last as well or better than NiCd and will have a lower lifetime cost because they don't require recharging as frequently (battery life is measured in recharge cycles) and don't self-discharge (which greatly reduces recharges required for batteries used sporadically).