Does anyone know of any good sites to download good design templates (master pages, css files) for MVC projects, or ASP.NET projects in general? I've used the asp.net mvc gallery but the options there are pretty limited. I'm willing to pay for some if they are good.
I second template monster. You're not going to find too many sites specifically targeting ASP.NET MVC like you're thinking.
Take one of those templates, edit it and stuff in some <asp:contentplaceholder />
tags and viola! An ASP.NET MVC master page is born... almost.
It's so easy to turn a simple HTML page into a template, you really shouldn't have too many problems. Once you start getting into it and run into problems, come back and ask more specific questions.
Good luck!
All of the best examples of websites built on ASP.NET are hosted on CodePlex. Take a look at BlogEngine.Net if you are wanting a good ASP.NET example website or if you are looking for a specific library or type of ASP.NET MVC site then you can use the sites search capabilities to find whatever you are looking for. CodePlex is the site for open source .Net technologies like ASP.NET and ASP.NET MVC. ASP.NET MVC futures is also hosted at codeplex if you are looking for additional features
I've easily fitted templates from the likes of Free CSS Templates in to numerous ASP.NET and ASP.NET MVC sites - they are very clean HTML and CSS templates, often built around common elements, so that once you've built your MasterPage changing to another one is often as simple as updating the CSS reference and changing some images.
Obviously there's a bit more work if you want to swap from a horizontal to vertical navigation, but it's not that hard.