Aether and Quebrith were the only 2 “alchemy ingredients” i was missing for a while. These ingredients are required for crafting some superior potions.
I discovered that the merchant who sells these recipes is Gremist, who can be found in Gedyneight, north of the Druid camp in Skellige.
You’ll have to fulfill his quest first which has a recommended level of 24 before he becomes an alchemist merchant. You’ll then be able to buy Quebrith and Rebis from his stock.