This has been offered in the past and is in cultivation as (staudtii X microsperma) - both yellow-flowered African species. However, it has been determined that this is not properly named. Until more is known, it is now being referred to as Begonia U690.
In any case, this plant is compact enough for medium-larger terrariums and it is stunningly attractive when full grown and blooming.
These are husky, established, cutting-grown plants, currently in 1 1/2" pot/cups.