Searching for Google Earth files is easy if you use advanced search and even easier if you learn a little about Google's search syntax.
Advanced search: It's right here...https://www.google.com/advanced_search
The key move on this page is the "file type" field: shift it to kml or kmz. Ta-da, that's it.
Using Google's search syntax: Just do a normal Google search but add filetype:kml or filetype:kmz in there. Look below: The left-hand image shows a search for "Revolutionary War"; the right-hand image shows the same search after I added "filetype:kmz"
Unfortunately, there is a lot of junk out there – I can help you find files, but I don't have a system for finding good files.