I don't know any in particular however I used to visit the Vogue forums for ideas and recommendations and heads-up on sales. If you're looking for a basic hat to dress up check out fabric & haberdashery stores like Lincraft and SpotLight. They sell plain hats for half of what you'd pay at a department store... but the decorations etc (feathers, ribbons etc) can be exxy.

My hint though is to start with the hat/head piece. It is often harder to find a hat to match a dress than it is to find a dress to match the the hat.