I do a reasonable amount of ASP.NET website or web application construction and have not found many websites that post sample ASP.NET masterpages or themes or skins. Generally, I end up modifying a sample css template. Here are some of the sites I have found useful.
One major difference about the templates is in how they utilize screen space. Some templates use only the center of the screen and would use, say only 50%, of a widescreen monitor.
These templates are free. Some require attribution.