Which thrift store has the best selection for semi-formal dresses?

I'm looking for a dress to wear to my wedding shower and wedding rehearsal. I hate buying new clothes and would like to find a dress at a thrift store if possible. I've already checked my favorites: the St. Vincent de Paul locations in Greenfield and Brown Deer and the Economy Center on Burleigh, and did not find anything I liked. What thrift store has the biggest/best selection for dresses? I'm willing to drive to a neighboring county if needed.