There's been a lot of interesting replies.

Quoting me saying that I already have a case, the problem was the cost of "real" C size batteries, then giving me a link to "C" size cases - not very interesting. Or useful.

The problem with the C or D batteries, as I mentioned, isn't one of availability it's one of cost. 4 real C cells cost around $40, 4 D cells $50-$60. Then you might have to buy the case. Then you have to buy the charger, because your standard charger nowadays typically only handles AA and AAA. By that point, you've spent the same money for a battery system that's larger, heavier, and far more annoying to charge than if you just bought the lith-ion version in the first place. Even replacement cost, once they run out, is the same.

P.S. Now I fully expect someone will post a link to the fake (same capacity in a wrapper shell) C or D cells and say that I was wrong about the cost...