Hello to the group. If the battery can't be found or maybe at a reasonable price, you can always build a voltage splitter. Opamp and a few resistors. Most of the meters I have?seen use something like 4 X 1.2V nicads in series center tapped.
Granted that means the unit has to be plugged in maybe filtering issues since that batteries isolated the system from ground and were the real filter caps.
But a consideration to return the meter to service.
I have done this years ago on a HP analog VTVM and its still works.