I have found some good hats on Etsy (GirlFabulosity makes a bunch of Kate replicas) and at some of the higher end department stores. The silly carts in the mall that have the fake ponytails sometimes have some good ones as well. I've also tried my hand at making some fascinators with sinamay bases and feathers, and always get lots of compliments on them. For our wedding last year, we encouraged our guests to wear hats- not everyone honored our request, but those who did looked so festive! Our photographer raved that she had never had so much fun photographing guests at a wedding.
I have to wait 24 hours to give you more karma.
I've started thinking about my Christmas hat. I love antique cut steel, and am thinking this could be a good way to put a button or buckle to use.