Looking for N-Channel MOSFET with igss<10nA
If you want to find a guaranteed low leakage- you may have to look through a lot of datasheets. 

CD4007 has a typical leakage of +/-10pA at 25°C

Note that most of the leakage in modern devices is the gate protection devices, not the gate itself. With a part like the 4007 you can connect the otherwise unused Vdd to double your anticipated common mode voltage to get approximate cancellation of the leakages. 

3N170/171 is available in SMT package and has 10pA maximum gate leakage (at 25°C) but expect to pay dearly for that kind of testing.