Focus Lenses should go to the Quills.

It makes perfect sense. They're aware of our Operators, and seem to know a lot about... esoteric stuff and even Sentients. Hell, they provide us with Amps tuned specifically for the Tenno! Of course they would have lenses available, or methods of their manufacturing that Tenno could borrow. They could even come from the Tower, or Unum herself, or perhaps excavated or found among other Orokin artifacts by the Ostrons which the Quills then take for the safekeeping.

With how essential lenses are for the Focus progression (you need at least one to even start earning Focus for any given school), locking them up behind RNG like this is criminal, and as much as i hate to say it, uninspired. RNG is utilized way too much in the game, and should be avoided whenever possible at this point, in my opinion. Putting something essential in a pretty diluted drop table of the mission that takes a lot of time and effort... it will make quite a few people mad, and reasonably so. Especially considering how much time it's gonna take to fully unlock even one tree.
That should be a concern for developers if they want Focus to be accepted by community. As it stands, community is quite divided regarding the subject.

So what i'm suggesting is adding lenses directly to the Quills that players will be able to buy for standing. Either fully built ones, or just blueprints (cheap if one-time use, more expensive if reusable). If blueprints, they will obviously require some Plains-specific resources, with higher tier lenses requiring rarer materials.

Regular lens is unlocked at Mote.

Greater lens is unlocked at Adherent.

Eidolon lens is unlocked at Architect.

If i could, i would change a lot more in terms of Focus lenses. Like getting rid of school separation and combining Focus XP in one common pool that could be used in any tree. Having to manage 3 tiers of lenses per 5 schools is goddamn annoying, it adds nothing positive to the game, and considering that Focus progression is already gated by the daily cap, it unnecessarily complicates already convoluted system and slows down progression even more. With this change you will be able to just freely lens all your favorite gear and play to your heart's content without having to worry that your daily Focus will go in the wrong tree.