I have to disagree with the Ann Taylor suggestions, but only because my style is a little more fitted and flashy than what I've found in their stores.
Just personal preference ... no offense intended.
I prefer H&M; you would be surprised at the great selection for suits--the materials aren't made to last forever, but the cuts are so much more young and stylish ... and for a great price!
My favorite designers are Theory and Dana Buchman. Theory has some nice outlets around the states, and I've found two Dana Buchman suits for over 70% off at Marshall Fields (ante-Macy's) and NM.com, respectively.
I've learned to always keep my eyes open and buy a suit when it fits well for a good price ... it sucks to be running around looking for a nice black suit hte day before you need it. My favorite black suit is a pencil skirt and fitted jacket from H&M.
I also like J.Crew and BR, but only at a steal on Ebay; otherwise, I consider the product overpriced for the level of quality and cut.
And if you're not too picky, Isaac Mizrahi at Target has some REALLY affordable and decent suits, although the cut is a bit more matronly than, say Theory.
HTH!!