There are those plants that one reads about and covets, but which never seems to be available from anywhere. Begonia bogneri is a remarkable plant from Madagascar, home to some pretty incredible plants as it is to better known incredible animals. Its grassy leaves, attractive flowers, and small size make it an ideal houseplant for avid plant collectors. I had an opportunity some years ago to get a single leaf to propagate from a friend at the NYBG, where they later sadly lost their plant when someone accidentally filled the glass bowl it was growing in with water one weekend so it drowned!
From a tiny, skinny leaf I have propagated several plants, and banked some seed in my refrigerator I set on the plants once they matured. If one is so lucky as to obtain a plant as rare as this one is, propagate it right away--its your best defense against losing it. Propagation is easy--cut leaves into sections a centimeter or so long, place on sphagnum moss (whole, not milled, live is even better but not necessary) in a pot in a zip lock bag under flourescent lights. They will root and grow small plantlets. I find the plants do better when transplanted into potting mix with some perlite once they are big enough--maybe it is because of the lack of nutrients in sphagnum. Sometimes older plants will appear to go dormant or partially so, losing leaves till all that is left is a greenish "bump"--the crown of the plant. It will regrow when ready, do not dry the plant out nor start to overwater it should this happen.
Begonia bogneri does best in a terrarium during the winter, but is fine outdoors in deep shade for the summer. As for any plant in terrariums, rainwater or distilled water is best, since repeated use of tap water will ultimately lead to salt buildup in the soil mix. It does not like getting overheated in an enclosed terrarium, as might be apt to happen during summer.
Once one figures out its needs, it is not at all hard to grow, so I am somewhat suprised that it seems to be utterly unavailable in commerce, at least as far as I am aware of.