i built a lot of my professional wardrobe from browsing at unique, a giant thrift store in merrifield, which you can get to by metroing to dunn loring and walking about 20 minutes (or drive). they often have really good quality secondhand professional clothes (ann taylor, ralph lauren, banana republic) for super low prices - you do have to dig because there is tons of stuff and a lot of crap, but i’ve often found lots of good professional stuff for 5-20 dollars an article