Did you already call to make sure they have one? They usually do not, but sometimes they find one somewhere.
You can also use Amazon.co.jp, which does seem to have them in stock. They don't ship internationally, though, so you have to use a forwarding service like Tenso.com. Basically you sign up with the forwarding service, which gives you a Japanese shipping address. Then you sign up with Amazon using that address and buy your product. Tenso receives the shipment and ships it to you for a fee (I think for me it was $30-40). That's how I got my eyepiece, the non-DS version of what you're looking for.
Since I already had that one bookmarked, I found the DS version for you
here.