Search for filetype:.kml or kmz gives you no results. So, do you know of any search string that will only give you only files to use in Google Earth?

1. Correct use of the operater is [filetype:ext] not [filetype:.ext] so it would be [filetype:kmz] or [filetype:kml]

2. The filetype operator doesnt work alone, you have to have some search terms along with it

If you do both of those, you will get some results. However, better way to find google earth files is to use a database such as http://googleearthhacks.com/