Thaler has the best voltage references -- there was a design in one of the
rags (Nuts n Volts or QEX) a few years ago in which 3 Thaler references
were placed in parallel for a super reference. Email me privately if you
would like a copy.

Also, get a copy of Linear Technology Application Note 86 "A Standards Lab
Grade 20 bit DAC with 0.1ppm/degC Drift" -- Jim Williams is the author --
and make sure to read the appendices. Williams also did an ApNote with a
comprehensive history of voltage measurement techniques, but I forget the
application number.