Yashica Electro 35 camera is a great film camera, but unfortunately the battery it uses (5.6v PX32 battery, also known as HM-4N) is not being produced any more.
Here are 2 practical solutions:
1. Use 1x CR123A+ 1x CR1/3N
Both the CR123A and CR1/3N have 3V in voltage. When they are stacked together, they have pretty much the same size as the PX32. The voltage you will get from them is 6V, which is close enough to 5.6v.
But most CR123A and CR1/3N batteries are one-time use, and they are not cheap.
The total cost of using a CR123A and a CR1/3N will be around US$5.
A cheaper alternative is to use 2pcs LR44 to replace the CR1/3N. But I would recommend using the CR/3N because it has the same chemistry as the CR123 and woudn't leak like the LR44.
2. Use an adapter+ PX28 battery (4LR44)
We are selling this PX32 adapter made of PVC and aluminum. It transform a 6v PX28 battery into a PX32.
PX28 is cheap and widely available. In the long run, the cost will be cheaper than that of the first method.
You can also use 4pcs LR44 button cells to replace the PX28 if you can't get it in where you live, since LR44 can be found more easily than the PX28.