It may depend on "where" you are shopping, but I bought the full ZWO NB Filter Set during the March ZWO Sale (which was offered throughout North America - perhaps further).
I selected the 36mm Unmounted set, as that would allow me to accommodate the wider Light Cone of Fast 35mm Photographic Lenses with my ASI1600MM-C.
Available were 1.25", 31mm, 36mm.
