If you have money to spare and you're looking for high-end designer couture vintage, I highly suggest stopping by the Vintage Couture section of 1stDibs.com
1st Dibs Vintage Couture carries designers such as Chanel, Emilio Pucci, Judith Leiber, Lanvin, Christian Dior, and Halston.
My favorites: (if I won the lottery I'd buy in a heartbeat)