It has been almost 10 years since I was hanging out there, but there were various shops in Akihabara. Prices were high on things there vs. waiting until I got back to the U.S. and then just bring it back over, or having someone ship it to me from the U.S.
Eye the specs rather well. Power limits as well as TX frequencies are different on some things in Japan. If you plan to use it there, you should be fine. Expect the manual to be in Japanese or not in English. Also, various shops may be closed on Sunday and tomorrow due to the Holiday. You may be better off shopping on amazon.co.jp and having it delivered if you need that radio while in Japan.
Rocket may have what you need or they can order it. Oh wow, their website notes that they are open today from 10:00 to 19:25 as well as holidays. Yeah, you may want to call first to see if they are open today and how long, especially with the holiday tomorrow.
I am figuring you are not licensed in Japan (MIC), or you would probably know where these radio shops are in Akiba. What you may already have and may be asked about is a reciprocal license for foreigners you would receive from the JARL. While I am not experienced on the exact rules, Japan is pretty tight on radio use. The shop may not, or may not be able to sell to you.
Hmm...I believe I understand where you are going with this via IRN. Yeah, that may work out fine. It seems slightly on the edge of RF rules as it pushes over WiFi, but again I am not knowledgeable on Japanese RF communication rules.
I hope you find what you need and it works well for you.
Good morning