I've never really watched FMA but you should check eBay for some cheap black dresses and modify them, like animenerd93 said. Maybe even check cheap retailers in your area, Kohl's, Charlotte Russe, Forever 21, maybe local prom stores? I'm sure a lot of their dresses will be going clearance soon. This might also sound odd, but you should seriously check out sites that sell stripper/club clothing, they usually have pretty sexy dresses which are affordable too.
Here are some links to some dresses that may work:
It may not be too helpful, but maybe it'll help you think of some ideas - good luck
Can't wait to see how it comes out.