First of all, this is very, very heavily based on this mod https://www.nexusmods.com/skyrimspecialedition/mods/954

However, it has a few issues due to its age:
It's form 43, not 44
It's missing an edit or two from official skyrim updates (that is, from update.esm)
It's missing numerous USSEP edits likely added after this mods creation
It has wild edits

Feel free to go to that mods page, and get a feel for the concept of what it (and this) does.

Done?

So, it adds 4 tiers of alchemical ingredients based on assumed rarity / difficulty to aquire. It also adds keywords to some ingredients, so you can buy them without the speech perk that allows the sale of any item to any merchant (Just Human Heart in this - the older mod added it to a lot, but USSEP has since done so)
It also adds Human Hearts and Human Flesh to Orc stronghold wise women, and Babette. Its not unreasonable for these vendors to possess these items.

I've made a few changes to the lists and the numbers based on my own thoughts and tastes.
I've removed the Crimson Nirnroot from being available to buy, IMO this shouldn't be available, it's meant to be so rare that Sinderion went to Blackreach for it!
I've also changed the numbers and the lists to control the numbers a little better. The chance none flag really does a number on what you intend to appear, so that got removed.
Finally, I removed the edits to the spouse inventory and the Skaal inventory. These IMO shouldn't be overflowing with ingredients.


There is also a couple of tiny patches for the Simon Magus mods Mysticism and Apothecary, which I truly believe you should at least consider using. I briefly considered only uploading a Simonrim version of this mod actually.

Anyway, check for conflicts, blah blah blah

If theres much demand for it, I could do a version where vendors have a lot more ingredient stock - this one is fairly tame in its adds

Credits for the idea and most of the implementation to cxgdarch