Get a friend/matronly figure who knows how to measure to measure you, then order a cheap thing from sears and it should fit decently well enough. My prom suit was a $100 thing from sears that was jacket, pants, shirt, and tie, though I wore a different shirt and tie beneath it.
$100 is not strictly trivial, but if you get something that isn't too showy, then you've got yourself a nice simple black suit to wear to fancy job interviews or balls or whatever. It's not a bad idea to simply have a nice suit jacket.