Secret Cow Level?

[NiteFox]

I have read that the other items are always found in the same places, but those places are dungeons that may or may not spawn in any given game. The plans are apparently in Izuel's drop-pool.

Apparently. I think the only certainty in the entire list is Wirt's Bell (Always available in Squirt's stock, but then again I hardly go out of my way to check her stock every time I'm passing).

Funnily enough, the only time I've seen the Black Mushroom was when I was running through Cathedral 1/NM with Crusader and specifically mentioned the item. It literally spawned about two rooms later.