The plain species should be the one in the database under Tradescantia pallida. Species is a botanical rank wereas cultivar (CULTivated VARiety) is a horticultural variation below the rank of species, to oversimplify. They come under different codes of nomenclature. The leaflet and booklet below may help.
http://www.hortax.org.uk/uploa...
http://www.hortax.org.uk/uploa...
I so agree with you about no single place to verify cultivar names. There are some databases but none seem complete. Some cultivar names would be very old also, predating the formal registers and the ICNCP. The single quotes should identify a cultivar as opposed to a variety (variety, or more correctly varietas is preceded by var. and the name in italics as opposed to Roman) but they are often used wrongly so can't always be relied upon. The RHS does verify plant names for their database, and Missouri Botanical Garden should also be a reasonable source to check.
