Not sure, as the meter is from the early 1970's it will take one of two possible batteries
PX625 the old 1.35 volt mercury battery, no longer available
Or
SR44, LR44 1.5 volt battery which you you can find near enough any where.
Other than that don't know.
I can find a few images of it on Google but none seem to indicate which battery. However a mooch around Ebay seems to suggest the PX625.
The 1.5 volt ones should work but you may have to compensate slightly because of the higher voltage.