HKJ did a review of “Tensai NiMH C-cells”
Test of Tensai C TR-C4000 4000mAh (White).

NKON in Europe sells NiMH C cells made by Maha and Ansmann.

Kaidomain sells Soshine C cell. These might be low self discharge (LSD).

Why not just use high quality Eneloop/Ikealoop AA cells in combination with a cheap AA-to-C cell convert/adapter, something like this or this ?