Will keep my Eyes peeled next time I am heading over to Kiya Hobbies, most of the HS close to me don't store much on AM parts for plane models, military & cars seems to be the big seller.
Also have a look at some of the Hasegawa and Fujimi Kits, plenty of with Japanese markings and some "US "kits were released in a smaller number with japanese insignia. I got the Apache with the japanese insignia.
Did you check Hobby Link Japan? They might have something on their catalog.
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Just did a search on Decal & Air:all looks like PLATZ and LifeLike Decals are your best options.
You can get to the PLATZ website via HLJ.
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Not Japanese but got myself the WW II Nose-art decal sheet, good stuff on there.