Here is what I use as my main voltage reference standard in the old 'man cave' here.http://www.ebay.com/itm/DIAL-A-VOLT-MODEL-DAV-46-STANDARD-VOLTAGE-/270874211378?pt=LH_DefaultDomain_0&hash=item3f115c7432
Of course most won't be as lucky to have picked one up very cheap at a surplus outlet many years ago.
There are many voltage reference ICs avalible that output a fixed voltage, 2.5vdc is a common value and are speced to certain guaranteed accuracy specs and only cost a few dollars. I've found that many digital DVMs are pretty accurate, but it's always good to have your own 'reference standard' on hand to compare unknown meters against. A .1vdc accuracy is not a difficult value to obtain with most modern DVMs.