If you don't have have a sewing machine then IDK how you could make one.
If you are REALLY desperate and really want a coat then I have few ideas:
#1 Fancy/Expensive-ish way
Buy fabric for it and have it custom tailored/made locally. If you choose that, then you have to be uber specific with what you want it to look like.
Get some one on here to commission the coat/cloak. probably around same price as above, maybe cheaper, depending on the person.
#3 OHCRUDTHECONISNEXTWEEK Cheapo way
What you could do is go to your local party city store, or store that might have left over Halloween stuff. Look to see if they have any Harry Potter robes, or cloaks that look similar. Then you can alter it to be more form-fitting/ add buttons etc.
BUT, there are a lot of times when she doesn't wear her cloak, and it still looks normal.
Personally I would just skip the coat until you could make one. ^^;
(Unless you happen to come across a matrix-like coat xD)
Hope that helps OvO