From all sources I can find, you should either automatically receive the mongoose recipe for the Oskmurk quest, or obtain it from Triss. Well, I'm on the quest to make the potion (on the part to craft it, in fact), and I do not have the formula. When speaking with Triss, there is no option to obtain it.

What am I missing here?

When you visit the Kayran's lair to collect sample, Triss will give you the recipe during the dialogue.

Hmm. Is it possible to accidentally sell it? and if so, is it possible to purchase it back? Or is it possible to get it by other means?

If you remember who you sold it to, there is a good chance you can buy it back. The quest can be done without it though. The Kayran I mean. You may also be able to get it from the female merchant in Lobinden or Cedric

If it is a quest item, I doubt you'll be able to sell it.

I am just curious whether the recipe itself is tagged as a quest item.

That was what I was referring to. I have seen some say that they went in there without it, but that Kayran, can throw out some large doses of poison.

Are you sure you don't have it?

I found that for some reason it was at the bottom of my list, and I had to scroll down to find it.

If you don't have it after exploring the Kayran lair with Triss, then you probably sold it by accident. If you haven't gone too much further, you can go back to a save just before then. If you have gone too far past, then don't bother. It is not needed for the Kayran fight, but you will not be able to get the "Strong Stomach" ability.

Sure, you can use it against anything that poisons... it has HUGE poison resistance. Typically, though - that is not what makes you lose a battle with a foe, so I doubt you'd find it very useful for anything other than the Kayran.

Actually against my fight with Kayran.. i didn't even get myself poisoned at all.. which kinda render the potion useless for me though.

Ok I just checked, the Mongoose recipe is not tagged as a quest item. So it is possible that you might have sold it.