Sure, why not?
I've designed instrument front ends using resistors as large as 50G,
which you can get from Digikey. (In fact they sell 500 Gohm resistors
in surface mount.)
One thing to remember is that they're very very slow--a 500G resistor
makes a 1-second time constant with 2 pF!
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com