I shells are the Infiltrator. You get one during the course of the story, then after the 2nd infiltrator mission to the Khrell Biolabs, you should be safe to give it to Nimhe for R&D and mass production. G shells are Golden Seductress, which are a rare bot container drop from Drengar Corsairs (the Drengar ships that don't spawn drones, and have a brighter yellow stripe on top). They show up in the QuNoth System (probably misspelled), and can occur from Bounty Level 6 missions, and random encounters from traveling encounters in QuNoth.
Bare in mind, both shells are Premium ones, that will add traits/attitudes to your bots, jumping them +30 IQ (for the Golden Seductress) or +70 IQ points, making them only really good for selling as Illegal bots.
However, I will admit that while it took me a long time of rerolling I was able to roll a 109 IQ Precursor Military/Elite cortex, then combine that with the best Elite grade parts and a Golden Seductress shell? Holy smokes, it's actually worth more than a sub 140 IQ Galaxina shell with best parts. Admittedly, part of that is because of an issue in v1.50 where the Galaxina shell's personality wasnt giving any sort of value bonus, so my statement may not be true in v2.0.