So I'm running my second play through, and I have still to find the base recipe for Hanged Man's Venom.

The enhanced recipe is easy to get (Greedy God Quest), but its kinda pointless if you cant find the base recipe.

Has anyone found it? and if so where?

I just noticed I found this potion recipe today. I'm not sure exactly where. It may have been one of the ones I bought from Keira, but I'm not entirely sure. I could also have just randomly looted it from a chest somewhere and didn't pay attention (I get so many).
Sorry, this may not be super helpful.

I normally find most recipes with herbalists. Some I find out in the wild but the majority I buy.

Unfortunately, I don't know where to get the base recipe. I'm having trouble finding White Rafford's Decoction, found the enhanced one, but never found the base potion.

I think it is down the trapdoor in Kiera Meitz's hut.

Yepp, it's in Keira's hut...

The herbalist with probably the most complete arsenal of recipes and other stuff is some kind of halfling living north-northeast of Oxenfurt (small hut, brightly colored).

I prefer to find the recipies. Buying them just gets you alot of "empty" (i mean useless stuff inside) chests at the end of caves, quests or in difficult to reach places. It simply removes the recipe from the chest if you already have it.