Could try Rapid Electronics at Colchester, since they have stocked the old 15v for some time. ( I'm going back a bit, but always found they lived up to their name)
15V? That was one of the batteries used in the AVO. IIRC it was used for the resistance measurement. Actually used 2 in the AVO
Never tried this, but was told that you could use it as a variable fuse (when doing current measurements). Just pop the button back in should it exceed the max