Top hat: check one of those Halloween stores for this. Just a note-- there are a bunch near me, and one had the same one for $20, and the other for like $6. So you may have to shop comparatively, but there are cheap ones around.
Blazer: black, unless you're going for a diff color scheme. You can wear a pre-preg one and leave it open maybe? Or borrow one from a (larger?) friend, maybe, since black blazers are pretty standard...
Button down- type shirt: white, or some other light/bright color, to contrast w/ the blazer. There are cute wrap ones for maternity-wear, I think. Maybe leave the top button(s) open or something for the sexiness.
Black knee high boots!
Pants that you can tuck under the boots-- skinny jeans, w/e. Cheaper alternative, esp if you don't want to invest in a pair of pants that might not fit later, may be to get a pair of black leggings, or hose or something, and formal shorts or skirt over that.
I'm a firm believer of crafting costumes out of Real Clothes, instead of buying costumes... so even if you do end up having to purchase a few things, hopefully you can incorporate them into your wardrobe later.