A quick search online shows you can get some sub C NiMH batteries around 5000mAh. If memory serves me correctly, I think the last NiMH I actually used was an IP 4200mAh.

That said... Good lord, why? I most definitely agree OffRoadJunkie , you can get much longer runtimes and performance out of LiPo batteries. They also last longer and are much easier to take care of. Yes there are downsides that I will not go into.