Appears to be 22 Netherbians in the caves , and on 1 clearing of them you will get apprx. 2 to 4 if your lucky, more times then not id get 2 blocks a clearing. very slow and boring as there isnt even anything else in zone for tradeskills to occupy your time. The Trogs and Fungoids in other caves dont drop anything of note
I've just finished a session of farming these, and went back to count. I estimate the drop rate to be about 1 in 6. (I got 21 out of 122 kills.) Unfortunately, people don't tend to hunt here much these days (unless they're farming for porous mineral blocks) so it's hard to find them on NPC merchants. It's going to be a long slow road to Tinkering 215.
This is now used in the Vah Shir Cultural Armor recipes for Journeyman level. And it's much cheaper and easier to browse NPC merchants for these(only 4gp+ each if I remember correctly....)
Not common drops. Much easier to buy them, but they're often priced rediculously high, as Geerlok Fermentation Devices are one of the best ways to get to 215, and cheapest, for Tinkering.