I asked my friend and she told me all she did was find a "puffy" jacket pattern and slightly modifiy it to have the right number of "puffs" to the sleeves and torso. She stuffed the "puffs" with fiber fill to make them stay puffy and she didn't bother adding a zipper since the jacket never really closes.
I don't know how much that helps you, but I wish you the bes tof luck.
~2015 Convention Schedule~
Anime Boston, Boston MA
CTcon, Hartford CT
Another Anime Con, Manchester NH
Ok, I saw this done at the Nintendo World B/W2 launch event:
From what I can tell, she just bought a puffy yellow coat.
"A philosopher will ask that if a tree were to fall in a forest, and no one is around to hear it, would it still make a sound. A smart person would ask why the presence of a person determines whether or not a tree makes a sound as it falls." - Soren Alenko