Getting the product into the market as a dropshipper is about targeting the most popular marketing avenues in Japan. Unfortunately, I'm not aware where most locals in Japan actually shop online. There is of course an eBay Japan Link hidden: Login to view but I have no idea of how popular it is.
It's no different to marketing a product in any country, you need to be marketing it in the right area to get maximum effectiveness.
Perhaps you should consider wholesale as well, maybe look for a local supplier/s to carry your products and retail them. Is there a particular reason you want to get into the Japanese market, are these shirts specifically designed for that market?
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