The B4/C4 flash (cube) adapter for Minox B was initially designed and used the Mallory PX825, cell battery. These are now extinct, because they contained mercury oxide.
Of course people needed batteries to replace this now extinct one, so several models were and are available on the market now. However, they may be scarce, but a good start on finding them should be here:
Models replacing the now extinct Mallory PX825
Type: Alkaline. Voltage: 1.5 V.
Diameter: 23.2 mm. Height: 6.1 mm.
You can find the battery here, last I checked they were still available: