I play with High Precision,Low Cost Current Sources Using the AD8276 Difference Amplifier and the AD8603 Op Amp
http://www.analog.com/en/circuits-from-the-lab/cn0099/vc.html
I want to try, source remake to picoamper (100pA) current source
Can recommend a suitable amplifier instead AD8603, appropriate transistor about JFET. I would welcome greater power supply for amplifier, but it will probably be a problem