... buy a DAC!
Thought I'd update this as SiliconWizard did make a somewhat important point:
The DAC80501 is an interesting product indeed, still not available (preproduction stage).
... right now the unit cost per 1k is given at $2.86.
...
As it's in preproduction, the unit cost may not quite be definitive...
These are now available and as yet the 12 bit DAC60501 is still priced at $1.14 @ 1k price but they are now also quoting $0.99 @3k. The bad news is that the temperature coefficient spec has increased from 5ppm/K to 10ppm/K. Dissapointing but perhaps not too surprising.
Also unsurpising is that the long term drift spec has also increased from 12 uV/1900h to 20uV/1900h. Perhaps they should include a long term drift specification for the drift specification...

But even 20uV/1900h is still hard to take seriouy - that's a mere 8ppm in 1900hs! Even the hermetic metal can version of the LM399 used in 6 1/2 digit meters including the 34401A was only specced at 20ppm/1000h! I don't know of any other (easily available) reference that has as good a specification other than the LTZ1000.
But still, they did revise the datasheet with this new figure so they must be making measurements - perhaps they really have made a ground breaking improvement in reference stability which they decided to showcase in a $0.99 DAC's reference!
The good news is that the 16 bit DAC80501 reference is still specced at 5ppm/K max; they are still $2.86 @ 1k but they are now also quoting $2.49 @ 3k which is pretty impressive.
So if you want a 5ppm/K max reference with a free - ish DAC it'll cost you $2.86 from TI - but bear in mind that the DAC specs are actually pretty good, especially the max INL at +/- 1LSB. Many low cost 16 bit DACs have max INLs of 16 bits or more - ie. 12 bit accuracy (or less).
[EDIT] typos