My unicorn is this one...
It's an Italian due-fogli (39x55") from Mario Bava's first film (a.k.a. 'Black Sunday'). Heritage has never sold one, neither has emovieposter.com, though Bruce has had a couple of French posters up for auction which utilized the same artwork.
So far it's been pretty elusive. KinoArt had one for sale when I first started collecting, but the price tag of 1700 Euro scared me away.
Since then, I have only seen it up for sale once, this spring on eBay. But as I wrote then in another thread, the seller ended the auction early and traded it privately for another rare Italian poster.
I have searched here and there and everywhere, but lo and behold it looks like I might actually be close to getting one now. I asked for it (again) last week on MoPo, and a seller wrote that she was actually offered it a couple of weeks ago and would inquire for me whether it was still for sale. And even if that doesn't work out, another seller emailed me privately that he might consider selling me his personal copy for the right price. "The right price" probably means I will have to take out a second mortgage, but frankly I don't care. I just want it.