This also drops off normal A Sensate Reishi, I'd call it uncommon although my sampling has been limited. I've gotten 0 from the Matures, and 0 from the juvenilles and under, although I might just not have had myluck with the A Mature Sensate Reishi.
The "Sensate Reishi Lens" is usually just sold as vedor trash. However, Tinkers can use them to make a "Geerlok Gem Setter" that will increase your jewelry skill by 5% while in your primary slot. You can often pick them up in the bazaar for about 5pp or just farm them in the Paludal Caverns.
Geerlok Gem Setter: gears, gem setter parts, gnomish bolts, reflective shard, SENSATE REISHI LENS and sprockets. I've seen the Geerlok Gem Setter go for 100-550pp ea.
Given the "List of Goods" items listed for her (linked above), I would BET she does not have in her standard supply (I also see random gems & jewelry, short beer, plauged rat tail, ect) While I really love having lists like this of vendor items available here, it would be more helpful to have only the items always, standardly available