Reflector is a quick win:
You can download the WPF control styles from my blog.
Or download the cool little util Style Snooper:
Show Me The Template is a tool for exploring the templates, be their data, control or items panel, that comes with the controls built into WPF for all 6 themes.
Or use Expression Blend by selecting an object in Xaml then from the Object menu select Edit Style > Edit a Copy..: