I am missing a few oils and potions but find myself with Crowns to burn. Are there any merchants that will sell oil/potion recipes?
You should look for the master alchemist on Skellige. His house (cave?) is located near the sacred druid grounds near two big oaks. You do a quest with
there. Also for him to sell you any ingredients or recipes you need to finish a quest chain involving some fetching things for him.
Source: Gosu Noob
I've played Polish version of the game and his name was "Gremista", so I expect it to be somewhat similiar in English - like "The Gremist" or something like this.