As to availability, it may help to consider it from the garden center point of view. Many garden centers and retail nurseries are simply resellers, not growing their own plants but purchasing them from wholesale growers and reselling them to the buying public. In many cases, the availability at GC's will be governed by seasonal considerations - the perennials they sell will be available on a rotating basis based on their blooming habits. Spring blooming perennials will be most prevalent in the spring, late bloomers in late season, etc. Because hellebores are fully evergreen in my climate, I do offer them year round, but by far the largest selection will be available in mid to late winter when the plants bloom. A blooming perennial will attract far more attention and customer demand than one that is not in its bloom season.

Unless you are located in an area that offers perennials on a year round basis (and many GC's do not) as well as one that carries an extensive selection of hellebores, you may be better off ordering a less common selection, like 'Silver Moon', by mail order.