This species was sold for a few years as B. tenuissima, but after some flower comparisons (I think by Maggie Hellis?), it was determined not to be a match.  The foliage morphology is incredibly similar, though.  More information on B. tenuissima can be found here. 

This species, whatever it is, occurs in Borneo as well, and is reminiscent of rheophytic Begonias like Begonia xiphophylla.  It is very slight and rambling, sticking close to the ground and hasn’t gotten more than about 7″ tall in my experience.  The leaves are about 1/3 inch thick and usually grow to about 2-2.5 inches. It seems to do best in sphagnum moss and can tolerate alot of moisture.

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