Question: Is there a good source to download free SharePoint Themes (.thmx) ?

Answer: 
Not free. <http://tuning.bind.pt/Sharepoint-Themes.aspx>
There is a vast quantity of web designs out there, but they are not designed for SharePoint. However, what you can do is render a page in SharePoint that will support some of these themes. So for example if you render HTML that looks like the CSS Garden HTML template you have a starting point to use their themes. But it's not going to be easy. Begin with a simplified [starter master page](http://startermasterpages.codeplex.com/), which is a normal practice for developing Internet sites on SharePoint anyway.

You are probably not going to get your HTML spot on without either using control adaptors and other techniques. So it is more likely that you will end up adapting the CSS.