I live in Sapporo and I'm in the same boat as you mate. Let's look at Burton Cartels. Stateside it's around 230 USD, in a Japanese retailer around 36,000 JPY (475 USD). Totally taking the piss!
I buy all my kit, apart from boots & helmets (stuff that needs to fit really nicely) online from US retailers. The problem is that only some of the retailers ship to Japan, and it's usually only certain brands. For example most US retailers I've dealt with have no problem shipping Mammut kit overseas, but I've never been able to get Black Diamond kit shipped out of the US.
Nonetheless, in the last year I've had 2 decks, 2 sets of bindings, 2 sets of pants, 2 sets of goggles, 1 shell, 1 fleece sent to my place in Sapporo in the last 12 months.
What I'd recommend is asking what retailers will ship to Japan, and if so what brands they'll ship.
Then again, if you're lucky and have a friend in the US just get the retailer to ship to your mate and have him fedex it to Japan.
Check out this thread for a few decent online retailers