Strict laws would suggest it's all got to be shipped in.
Japan itself would appear to have a limited number of homebrewing suppliers - much less the island of Okinawa... somethin to do with the fact that brewing anything over 1% ABV (that's one percent) ... is technically against the law.
Good info here tho .. check this out ...
Beer in Japan » Homebrewing in Japan
Info on CO2 in that article as well.
Ganbatte Kudasai!