Recommended G-Shock Dealers in Japan
From G-Shock Wiki | casio watch resources
These are the big three Japanese dealers in Japan. They are all very reputable and fast. Each offers great service and is popular with G-shock collectors in the US, Europe and other countries.
Chino - http://c-watch.co.jp/
Higuchi - http://www.higuchi-inc.com/
Seiya - http://www.seiyajapan.com/
Each of these guys are used to dealing with buyers in English. And they do have request forms for models not listed on their websites. Just be respectful of their time if you have any questions.