Cosplayers! I need help!

So I really want to cosplay Elsa from Frozen. I've wanted to since I first saw the movie. I've just been having problems finding the right costume. I'd make my own, but I don't know how to sew. Also, I'm a plus size cosplayer, so when i find a costume online I have to make sure it either comes in my size or can be custom made. I've searched all over the internet trying to find that perfect beautiful icy blue dress, and there are a lot of them out there. The only problem is I'm worried that when the costume actually arrives on my doorstep it'll look awful and I'll have been ripped off by some online seller.

So my question is, does anyone out there know a trustworthy place online where I can get an authentic Elsa dress that can be custom made?