I don't work in an office, but I have to dress professionally and in all black and/or white. I shop at Target, Forever 21, Express and New York and Company mainly. I'm a hairstylist, so I don't like to spend a ton of money on clothes in case I get color on them.
ETA: OH! And H&M... love me some h&m.
Target and Dillard's Clearence. I got a ton of office dresses from target for $4 each on Clearence.
Kohls, JC Penny, Macy's, KMart. Mostly, though I shop second hand.