itch.io

When the shopkeeper gets sick, you need to 'go back home' and 'study' about the symptoms. Then you go to the swamp, ask the Priestess about Heat Occasia then you can get the recipe.

There are also fanmade wikis around (although they're not complete) but I got advanced recipes there even before it was introduced in my current quest. If you try crafting them early, they'd be added to your recipe list in advance.