Buy Japanese High School Uniforms
It can be hard to find genuine Japanese high school uniforms that are not just meant for cosplay. Whenever you go to an anime/manga store, you can find a variety of different costumes that take the appearance of high school uniforms—but they're just costumes. They're not real high school uniforms.
But if you are looking for authentic Japanese high school uniforms, you've come to the right place, because a famous Nagoya-based company called Matsukameya offers tailor-made sailor uniforms (both winter and summer versions) for foreign buyers at a very reasonable price.