Precision 1nA - 10mA current source
These circuits work well, I've built several variations of them to deliver accurate amounts of current, but mostly on the load side. I have a few suggestions:


Use a low noise amplifier
Use a low leakage mosfet
Make sure the feedback loop is compensated properly, the mosfet has small amounts of capacitance and can create poles in the kHz to MHz range. If the poles are higher than the DC gain, then this can create a resonance point and causes oscillations. The best way is to simulate in SPICE with different DC levels while running an AC analysis to ensure stability. 
Make sure you use a low noise DAC
If you use a MUX put the feedback loop so it includes the mux (this is a big hint).
Instead of a mux, if the leakage could be controlled, you might be able to use two circuits in parallel.




The circuit above simulates down to 1nA with a 1mV input.

If you need absolute calibration to a specific level of current, I'd buy a Keithly current source meter (like the 2400 Keithly SMU)