There are 2 major Nishikigoi organizations in Japan, Shinkokai for breeders and dealers, ZNA for hobbyists. Both of them are trying to spread Japanese Koi culture, please contact them if you are interested in joining them.
All Japan Nishikigoi Promotion Association
Organization of Nishikigoi breeders and dealers. Established on January 1970.
174-1 Hiu Otsu, Ojiya, Niigata 947-0003 JAPAN
Niigata District https://jnpa-niigata.com/
Chugoku District https://carp51.jp/
Shikokai Koi Show Results https://koishownews.koiwork.com/