This is an unidentified species to my knowledge, and new to cultivation. I have not seen it offered anywhere else in the US.  It is from Borneo and has a beautiful ferny shape. It is highly iridescent, almost black in certain lighting (as black as Begonias get, at least) with bright red abaxial (backside) coloring.  The petioles and central leaf vein are also red, and the petiole and stem are covered in tiny red hairs all over.  It is a slower-growing species but has not been particularly finicky in my experience.  Like many Bornean species, it can melt quickly if conditions are not to its liking.

The last photo is representative of the plants available.  They are well rooted and shipped meticulously in their pots.