http://www.voltagestandard.com/ sells simple and inexpensive voltage references.
They currently offer:
- 10V 0.003% for ~$50
- 2.048V, 2.500V, 3.000V, 4.096V, 4.500V, or 5.000V 0.01% for ~$30
- choose-your-own 5 voltages up to 0.01% for ~$90
All references come with calibration certificate and include free re-calibration for some time.