Admittedly this thread is from last year but I'll bite.
When an overseas cosplay seller refers to "uniform cloth" as the fabric used for their costumes, what they mean is a generic poly/cotton blend of low quality that can be used for almost anything in mass produced garment-making. It's not what actual uniforms are made out of, certainly not military or school uniforms.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com