I don't get it. I love to dress up for carneval.
The trick is to have one, or two boxes with stuff for costumes you can mix and match. Sunnglasses, gloves, assortments of hats, glass balls, whatever. And of course: face paint.
Then, when it's 5 minutes before the party: improvise.
Last costume party I went to, I was a "hacker". I put on a hoody and painted on a guy fawkes mask. No costume shop required.