I have just bought loads of equipment, and hunted out the cheapest international deal to do so. Here are my findings.
The cheapest site on the internet appears to be www.leisurepro.com
Postage is incredibly expensive to Japan, mainly because they use FedEx Express which only takes 48 hours. I ordered 2 full sets of gear and paid 177USD postage. If you contact them privately, you may be able to negotiate a cheaper shipping option. You will also be charged an import tax if the value of the items is marked on the box and the content is not (water sports equipment should technically not be taxed). Again, you might be able to get around that if you email privately, but I was only charged 3,200yen on a 140,000yen order.
Regardless, even with postage and customs the prices can't be beaten in Japan.
One tip, though - if you are ordering several items, then your best bet could be to use their "Create Your Own Package" feature. The set packages can be very cheap, but prices fluctuate almost daily. One day the Mares #2 package will be 671USD, the next day 911USD. The jumps are that big. So, either make your own or keep an eye on your favourite package over time.
- Although you can take air tanks on aircraft, they need to be prepped for inspection (not sure how that would work in this scenario, but it's probably not an issue if they're new), and you may find it easier just to rent or buy in Japan.
- It's also worth checking LeisurePro's "used" section, which seems to mostly consist of ex-display models with big discounts.