Amazon Japan ships to the US, and I assume to most other places where Amazon does business. Even if you don't know Japanese, it's fairly easy to purchase stuff from Amazon Japan - when you go to the website, you should see a link that says "in English".
Click on it, and most (though I don't think all) of the interface gets changed over to English. Product descriptions and stuff don't, though.
Once you've done that, you'll need to make an account, even if you have an account on Amazon US or Amazon UK or whatever. The process is the same as making an account on any other Amazon site.
The product you want to purchase is either this one for BD, or this one for DVD. In either case, you can pre-order it. If you do place a pre-order, the product should leave Amazon's Japanese warehouse on release day and get to you probably less than a week later.
As far as I know, there is only one option available for global shipping from Amazon Japan. You will be charged probably $10-12 for shipment to the US. I don't know about other countries. If you are in the US, you should choose to pay in USD, so as to avoid foreign transaction fees on your credit card.
Incidentally, you can also preorder the second movie (BD, DVD) for when it releases on 2 Apr 2016.
Much of the advice above is generally applicable to purchasing any BDs and DVDs from Amazon Japan. Amazon Japan isn't always the cheapest option, but it is definitely the most convenient, at least for US-based buyers. However, if you are planning on purchasing a large number of items in a short period of time, you may be able to save on shipping by using a proxy service like Tenso. (I assume there are other such services, but I haven't really looked.)