Micro-Voltage Reference
Yes, use a voltage divider. For example, you can get 10 ohm 0.1% resistors for &lt; $1. Then you can divide down (say) a 2.5V reference to 25mV or whatever you need. It's possible to use less than 10 ohms for the low side of the divider, but then you would probably need to use four wire Kelvin connections to the lower value resistor and the output would not be ground referenced and the reference  have to deliver too much current. 

You can parallel a precise resistor with a similar (or perhaps lower precision) resistor to get a precise ratio. For example, to get 25mV from a 2.5V reference you could use 10 ohms 0.1% (low side) and 1K 0.1% in parallel with 100K 1% (high side), to get 25mV +/-0.2%. 

For uV accuracy, take care that the layout and thermal conditions are well controlled. Just a 10m ohm resistance in series with the 10 ohm 0.1% will completely destroy the accuracy and that's only about 1/2" of relatively wide 0.05" 0.5oz copper. Similarly, keep the resistors away from any heat sources and try to keep them shielded from air currents (more because of thermal EMFs than because of the temperature coefficient of the resistors). 

The resistor noise should not be much more than the theoretical minimum Johnson-Nyquist noise - about 90nV RMS over a 50kHz bandwidth for a 10 ohm resistor. 

In addition to input impedance, which you say is relatively high, pay some attention to bias current. It's possible to have a high input impedance amplifier with a relatively high bias current, since the bias current may be relatively constant and impedance is an AC characteristic.