On the homepage of the International Society for Horticultural Science they offer for free a download (PDF-format) of the ICNCP (Scripta Horticulturae #18) - you can also find an orderform for buying it in hardcover and support their work:
https://www.ishs.org/scripta-hortic...-nomenclature-cultivated-plants-ninth-edition
https://www.ishs.org/scripta-hortic...-nomenclature-cultivated-plants-ninth-edition