For those who are interested, I was to be able to visit Japan last December. During the trip I was lucky to find many different Japanese citrus.
Yuzu harvest season had just ended. You could find yuzu at any major grocery store, and often Kabosu and Sudachi as well.
Premium yuzu at 3€ each
Kabosu grown in Tokyo. The owner, an old coffee lady, insisted that the flowers attract many swallowtail butterflies each year.
In Tokyo I found many antenna shops. These shops represent a single prefecture. A great spot to find many different citrus-related products and fresh fruits including Jabara citrus from Wakayama, Yuzu from Koichi, Kabosu from Oita, Shikuwasa from Okinawa and Sudachi, Yukou and Awa Suzuka from Tokushima. Awa Suzuka is apparently a hybrid between Sudachi and Yuzu. It was recently released and Tokushima is currently trying to promote the citrus. I found some more information here:
https://4citrus.com/english/ . It was quite an impressive fruit.
Dekopons were promoted a lot! Very high quality and tasty fruit, but often priced at at least 6€ per fruit, often double of that. I did look like there were different varieties of dekopons, since the I saw different sizes and colorations of the fruit. But this could be an other class of dekopon.
I visited the Koichi prefecture to look for some Yuzu farms. I mostly saw Citrus being grown in plastic tunnels. In the mountain region of Kochi, Yuzu was instead mostly grown unprotected. It was pretty impressive to see the scale on which yuzu was grown. Almost all of the yuzu I found was the very seedy kind. In fancier restaurants, I was sometimes met with seedless yuzu and sudachi. Unfortunately I have not found any seedless varieties in Europe. In Japan, it was simply available on Amazon.