https://usa.visa.com/Forms/merchant-...aint-form.html - use the 'other' checkbox.

But be sure that this is actually a DCC situation. If they process all cards through a merchant account in, say, the US, they can still charge you USD even if your card is in, say, CAD. This would only be against policy if the site shows prices in JPY but then actually charges you in a different currency.