Here's a few shops in Japan, but they are only in Japanese.
www.y-bonsai.co.jp
shop.bonsai-shofuen.com
www.bonsaimyo.com
greenscape.co.jp
www.yamataya.com
asahi-bonsai.com

Wow, Persimmomn, and thank you!
I had not found one of them while searching myself.

Here's another one:
www.bonsai.co.jp
Just placed an order there, and let's see how it turns out...
Actually, thank you Jens for starting this topic, because it prompted me to order my stuff online instead of spending all day sitting on trains back and forth to Saitama Bonsaimura

Back to your original topic: In addition to the links I have put here, there are tons more in Rakuten (rakuten.co.jp, NOT rakuten.com). Of course, everything under Rakuten is (more or less) exclusively in Japanese. Many of the shops on Rakuten are small independent businesses, and probably that is where you could find partners to get sales started in Japan (?)