It's just random drop like everything else.
Not my video: https://www.youtube.com/watch?v=f4NiBh4eI6k

I have found five Heartseekers with Sorcerer. It's not that diffucult. LT runs does not pay off, it's better to clear normal levels. If you go for LT it's better to kill mages down there. Better than chest drops.

Maybe you did not find recipe yet?
https://ibb.co/f8bbtWG