I'm not sure how big it is in Japan. I will say that, during my internet surfing/searching, I've stumbled on a ton of Russian/Slavic sites for cross stitch and a lot of French Blogs and Chinese sellers of supplies, but I don't remember coming upon any Japanese websites for cross stitch. I have seen sashiko, surface embroidery books and a couple Hardanger books in Japanese though. I don't know if DMC thread is regularly available, but embroidery floss shouldn't be too hard to find. I've even seen some at DAISO Japan stores in the states. I do know that the metallic DMC Diamant line is made in Japan also.
Perhaps you should look for Japanese blogs and websites for cross stitch. Maybe they'll tell you what brand is commonly used (anchor/dmc/madeira etc) and where they can be found. I think it's クロスステッチ for cross stitch and 刺しゅう for embroidery. My own Japanese is not good enough to try reading any webpages myself.
also this may be an old site but try checking DMC's Japanese site
http://www.dmc-ja.com
If these aren't successful/helpful, I supposed there's always the alternative of kitting up a year or so worth of projects to bring over. Some skeins and fabric shouldn't take up that much space in your luggage.
