Mike
I visited Tani Koi Farm with a couple of mates when we had a spare afternoon around Narita airport.
It's a reasonably large typical western aquatic outlet, with a range of different Koi on sale.
Getting to it is fairly simple, just get a train from Narita Airport or Tokyo stations that stops at Narita (town not airport) and then get a taxi from the front entrance. It took a bit of discussion between drivers to establish exactly where we wanted to go but was no problem.
If you e-mail Tani they will reply in English.
If they do visit then make sure they visit the back greenhouse where the better Koi were housed on our visit, many of which had just returned from the AJS.
More info can be found here.....
English Page
The Japanese map would be useful for taxi drivers I guess.
ݒn}
Be warned the blue breeders book has lots of red herrings for dealers in Tokyo. At least 2 are listed for Shinjuku, both in large department stores, neither of which are seemingly there anymore and I suspect were little more than pet departments.