There were a few threads bout those Chinese references: most of them seem to be reasonable. The best chance to get a good one would be to check something like this forum for other peoples results (those who can check) and buy from such a source. Also getting 2 from separate sources and a cross check might be good enough. If the requirements are nor that higher - e.g. more like a crude check on analog and maybe 3 digit meters, an of the self voltage would be Ok, but even those boards with fake reading would meet that low limit, as the refs used are also usually reasonably good.
To really do a calibration on a 4.5 digit meter the reference should be considerably better than the meters specs. So more like 0.01% level - few chips are available at this level.
Another practical way at this very low level would be to buy a new DMM every 1-2 years and use that calibration as a reference point. At something like 2 V or 20 V even the cheap meters are usually relatively accurate, often better than a similar priced reference chip.