I wouldn't dream of buying a "costume" pre-made. Like, I've seen a bunch of Sora, Roxas, or Xemnas sets.
But I'm not big on sewing either. I've been buying the basic items seperately, then altering if necessary. Thankfully my costumes require relatively little altering. Like, my blazer for Kasumi from Rose Hip Zero- I didn't see the point in making such a basic piece of clothing. I found a $10 white one in my size & I'm dying it. "Eternity Fashion" here in Philly has some pretty crazy styles, and I was able to find a $12 dress exactly like Marvel Girls from X-Men. Jesse from Pokemon