Just use a cardboard box from the bike shop, they should give it to you for free and you can even ask to take a look at how the bikes come shipped as it can be confusing figuring out how to get the bike in there if you have never done it before. I would even take a couple pics if it will help you remember.

remember those bikes have came to the US over just as far as you plan to ship your bike with no issues. One really important thing is to make sure you put the spacer in the fork to keep from bending the legs in, ever new bike ships with one so if you ask nice the shop will probably just give you all the packing material off a new bike they are about to build and you should be set.