paszekj wrote:If I purchase a reg from the states, will it be compatible with a CO2 tank here?
No, the threads on the CO2 tanks are different and the US regulator won't fit.
Or, if I get a tank and reg from the states, is it possible to get it filled here?
Again No, the gas companies here will not touch a tank from the States. Japanese regulations are different and any tank that didn't start out under the Japanese regulations isn't accepted.
Any advice would be greatly appreciated. Also, if anyone knows how to say "I'd like to rent a CO2 tank" in Japanese, I'd be really impressed.
Cheapest solution is to go to a local liquor store and ask for a CO2 tank -- it should be about Y5000 for deposit and Y2000 for a fill (5-kg tank). Although my Japanese is pitiful even after many years here, I can usually get the shop to understand by saying something like, "Tansan gasu bombe no rentaru dekimasuka?" which literally means "Carbonation gas tank rental can you do?" They should also rent regulators (sometimes they understand the English word, but sign language works well for that one

) for an additional Y5000 or so. When you take the tank and regulator back, you get your Y5000 each back as well -- or you can pass them on to another homebrewer when you leave.
One thing worth mentioning is that the tubing you'll need is very narrow (see pic below) -- one end just slides into a hole on the bottom of the regulator, where it gets locked in (there's a retaining ring that you have put pull up to release the hose), and the other end will barely squeeze onto a barbed quick disconnect, like
this one:
To get the tubing onto the barb, you'll have to heat it with boiling water and then twist and turn and jam it on. Once you get it on, it ain't coming off! So make sure you get it right the first time. And once it's on, there's no need for a clamp or anything, so don't worry about that.
You should be able to get the tubing at a large Home Center or DIY store or any of those places that have supplies for those do-it-yourself types. It's not always red like in the first picture below -- sometimes it's a kind of translucent green like in the second picture or maybe even black. I'm pretty sure that any tubing that will fit in the regulator is CO2-safe, but the best way would be to take the regulator to the store and ask for tubing/hose that fits (I think the word "hosu" will do the trick).
You can see the regulator and tubing here.
Here's the tank next to a 5-gal corny (behind) and 3-gallon cornies (to the side) to give you and idea of size.
Hope that helps.