The direct link to the WPF themes project is here: WPF themes
Download the source code (currently there is no binary release) and check out the demo that comes with it to get an idea of the capabilities.
You will need to install the WPF toolkit in order to compile and use the themes.
This is an ongoing project, so I think more themes will be added in the future. It will probably ruin the the sites trying to sell themes to you, but it is great for one man shops that can't invest too much up front.