I use AA->C or D converters like the ones Chet linked to for some LED lanterns. They work well, just be aware they will last 1/6 to 1/8 shorter than real D batteries.
I have used energizer rechargeable before, but they seemed to lose the charge within a week, even with no use. We were using them for my nieces baby toys when she was younger, can be a life threatening situation when you find out her favorite distraction that makes her stop crying isn't going to turn on any time soon.