I know this is an older thread, but I want to provide an update for those who face the same issue as me of setting up a home kegerator system.
So, as this thread stated, there is difficulty getting a U.S. regulator on a Japanese CO2 tank. The reason being that U.S. regulator valve bolts are CGA-320 standard (approx. 22mm) while the Japanese regulator valve bolts are JIS B 8246 standard (approx. 1/2 inch). That being said, once I received my own regulator from the U.S., I had to figure out how to attach it to the CO2 tank I rented.
Bottom Line Up Front: The bolt from a 15.8 x 1/2 M154RK coupler does the trick. If you are in the Zama/Atsugi area, take a trip to Handy Home Center (http://www.handy-hc.com/) and pick one up there. It is on Highway 42 on the left hand side while driving toward Fujisawa. Nearest station may be Yamato Station (Kanagawa), but you'll need a cab from there. The address is: １７７７ Fukaya, Ayase, Kanagawa Prefecture 252-1103, Japan
Since I am stationed at Zama, I drive by it every day when I go home. I saw it and decided to try my luck and go in. While browsing, I found of couplers next to their alcohol section (nicely placed). Doing some trial and error, and convincing a worker to escort me to my car so I could test a bolt on my CO2 tank. It fit perfectly, and 1400 Yen later I had the piece I needed. Steep price, but meh, my kegerator is complete so I can't be mad.
I am not sure what other stores would have this, but if you are not around Zama/Atsugi (or can't make the trip), print out the attached picture, go to home stores and browse around until someone says they have it.
Alternative options I have come across but did not pursue at this time:
1) Welding shops. Hard to find, especially if you don't speak Japanese.
2) Most Japanese bars get their regulators from the major brewers, so they are not a good resource for tips unless one says they can hook you up with a Sapporo/Kirin regulator. If you live on a military base, go to your local base bar/club and talk to the bartenders (or the MWR bar manager). They can likely give you some advise on finding what you need since they are typically Japanese locals and know the difficulties working with American products and Japanese market.
Best of luck to the other Geijun kegsters out there. Cheers!