Where do I find a formula for the Necrophage Oil in Flotsam?
Solution 1:
I've found the place to get the formula. It's at Anzeka, the same person who offers to buy off Newboy's Amulet from you if you have it. It's in the village outside of the town, the merchant icon furtherest to the right
Solution 2:
I do have it and I am still in Flotsam, but I don't remember where I've got it. I did explore pretty much everything, including the Nekker caves, the cave under the elven bath and the asylum. If you can't find it on a merchant, check these locations (there were several chests/corpses with "special" loot in those).