Hi there,
This company makes exact sized repro batteries using alkaline cells to replace the forbidden Mercury cells.
http://www.exellbattery.com/
I collect Standard Radio of Japan miniature FM sets and they all used RM-640 Mercury batteries. Took me 3 years of searching on & off before finding this co.
If it is the 640 type you need, the Exell replacement is A640/1,5V/350 mAh
Size : Height: 11,2mm /Diameter: 16mm
On site ordering, will ship anywhere in the world.
I have no commercial ties to them.
Best Regards
jhalphen