Requirements for low voltage, low current MOSFET used in current source
I think I would be looking at low capacitance small MOSFETs like the 2N7000/2N7002 - you don’t want your buffer to add more capacitance than what you are driving. There are others in the BSS series. 

Look for low Id parts and fairly high voltage rating (like 30-80V). 

A lot of the physically small parts are low voltage high current designs that can switch amperes and have metric tons of capacitance.