There are no longer any batteries on the market with a
native voltage stable enough to use as a reference.

Yep, istr that there was one type of mercury battery which could indeed be
used as a reference cell, but nowadays
they cannot be made because of disposal problems. Perhaps they could be made
under the stipulation that they be made available only to technicians or supply
houses, so that the general population would not have ready access, and thereby
not contribute much to pollution
(ok, so that's a pipe dream... ;-) ) -ph