Hey, I'm actually making a Black Rock Shooter cosplay too. I made my own pattern- I'd actually start by buying a type of "pattern paper." I'm not quite sure what it's called, but it's a cheap fabric that is easy to write on.
I made my pattern around five pieces- two sleeves, a back, and two front pieces. My sleeve pattern is based off this tutorial: http://www.craftstylish.com/item/786...tern/page/all/
The base of my coat is based off a fitted jacket that I own, and I just extended the bottom of the coat down to about my calves. Make sure to take careful measurements and draw careful lines! It's pretty easy if you have a jacket, but otherwise measure your shoulders, waist, basically everything.
The hood I'm not too sure about, but I think it'd be pretty easy to just Google that.
Hope I helped!