Yes, Shimpaku is Juniperus chinensis. But, I’ve understood it to be a “locality” - a variation of the species occurring only in a specific location, which in this case is Japan. I don’t think we would consider it to be a cultivar, for one because it isn’t necessarily an anomaly in nature like the many varieties of JM - it’s just the form that the species takes in a particular region in Japan. Secondly, there are two (at least two) cultivars of Shimpaku; Itoigawa and Kishu. Are there cultivars of cultivars? < actual question, not being facetious. I would assume no, any variation of a cultivar would just be considered a separate cultivar.
So Shimpaku is J. chinensis, but it’s not a cultivar, it’s a Locality. Therefore it naturally reproduces in nature. So based on that, there must be Shimpaku seeds...?
Binomial nomenclatures:
Juniperus chinensis ‘Shimpaku’
Juniperus chinensis ‘Shimpaku’ ‘Itoigawa’
Juniperus chinensis ‘Shimpaku’ ‘Kishu’