If you have a 1.5V battery, connect the output leads of a 100k potentiometer across it; take one of the outer leads and the wiper lead and this gives you your variable voltage source from 0-1.5V. In series with the wiper lead, connect a large value resistor (or large value variable resistor if you can find one) - this allows you to set the current.
e.g. if you adjust the voltage with the first potentiometer to 0.5V, and connect a 10Megaohm resistor in series with the wiper, you'll get a maximum of 0.5V/10M = 50nA.