Begonia handelii var. prostrata
A native of Southern China and Indo-China, this Begonia can grow to over a foot tall, with leaves staying around 4-5″. It is very easy to keep small in the terrarium however, and seems to be a slower growing, albeit not too difficult, species. The stems are a striking red and the white spotting on foliage becomes even more prominent as the plant matures.
The last photo is representative of the plants available. They are well rooted and shipped meticulously in their pots.