I wouldn’t bother looking for ASP.NET stuff specifically (probably won’t find any anyways). Finding a good CSS theme easily can be used in ASP.NET.
Here’s some sites that I love for CSS goodness:
http://www.freecsstemplates.org/
http://www.oswd.org/
http://www.openwebdesign.org/
http://www.styleshout.com/
http://www.freelayouts.com/