I've heard that people who attend anime fairs make their own. You can make a sailor fuku easily (and cuter!) by obtaining a pleated skirt and sawing a collar on a normal shirt.In any case, if you're talking about REAL school uniforms, I'm sure you know that any uniform is only for members of that group (that's what uniforms are for, to identify that you're part of that group). So basically you're not supposed to be wearing them. Second hand uniforms are sold inside that school or between schoolmates.
However, uniforms do exist for anime fans and those who enjoy sexual cos-play. The black market second hands are designed for pervs who prefer uniforms that were actually worn on real girl's skin.
Good luck. Maybe you can find something through an anime related link.