I know Mary Catomy's daughter and Mary does FANTASTIC work...she also books up like CRAZY. March might be too soon to be able to use her. Call her anyways and see though, it's wedding "off season" so it might work.
My bridal alts were through the place I got my bridesmaid dresses, Marrilee Bridal in Arlington Heights. I'm not sure they do alterations on gowns not bought through there, but you can check? They let me do it because I bought 8 BM dresses
there, and they did FANTASTIC work. And...in 24 hours, because their seamstress was going out of the country.
Good luck! What specific area of Chicago are you looking in?? I can ask around...