How can I find oil recipes in The Witcher 3?

I'm playing the Witcher 3. This is my 1st playthrough. I've finished all of the main and side quests in White Orchard and Velen. Now, I'm doing Novigrad quests.

Where can I find the missing oil recipes? I have searched in herbalist shops, and I couldn't find any of the oils that I'm missing. For example, I couldn't find the Widow oil. I do have many oils, but I'm missing a lot of the main monsters oils, too.

Solution 1:

Alchemy recipes could be found as random drop in chests around the world - just explore points of interests on the map (bandit camps, guarded treasures, etc.) and you will get most of them eventually.

Some recipes could be bought from vendors - see this question or this guide. I generally just check every vendor I meet - it's also helps if you are aiming to get achievement for all gwent cards.

And finally, recipes for quest items are given to you automatically when you need them (starting the quest or talking to someone).