What kind of material are you going to use for the jacket If you use something too light it will sag so I suggest you inveest in good quality unifrom material.
Don't try to alter someone elses unifrom because that is a lot of work. More than you really imagine. Also for the the collar you need trim don't try to use binding tape it will come out uneven.
As for the pattern try a pattern from McCalls or Butterick in the historical section. They should have one. However don't follow the pattern to the stitch because they are usually too big. Just get the general idea of the form and then use a suit jacket or a tight fit T-shirt as your guide to your size. Of course you should do the basic measurements of yourself first.
Or you can just commission if they turns out too complicated.
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