From Linear System's datasheets, assuming this is a most important spec for electrometer use:
IGSS gate leakage current:
50pA 2N5911,12
3pA SST/U424, U425, U426
2pA 2N5905,6,7,8,9
1pA SST/U421, U422, U423
0.2pA LS830, LS831, LS832, LS833
I thought Keithley part number TG-98, is Intersil ITS30092 Dual N-ch JFET TO-71.
I did find GE/Intersil IT500 dual cascoded N-ch JFET, a real odd part, but high IGSS 100pA.