Not sure if you have one close to where you live but Boundary Mill is a fantastic place to pick up really nice/special evening wear at reasonable prices too! Otherwise agree that the web (eBay etc) is a really good place to try, I've had loads of gorgeous dresses from online shopping ... really easy to return these days if not right for you. 
Seems it’s now Harry’s uncle - the Earl of Spencer - having his turn at a “tell all” book.




