You certainly came to the right person. Check out my list on ThisNext which has pretty much everything Blair has ever worn on her back!
Gossip Girl Fashion: Blair Waldorf
Since this is for a costume party, I wouldn’t advise spending as much as these outfits really cost. Head over to a school uniform supply store for the basics (skirt, tie), go to Target for the tights, and check out vintage stores or cheapy stores like Forever 21 for the pieces you can’t find at the school uniform store.
Good luck and send me pictures of your final look 🙂