A compiled list is at http://placefiles.iawx.net/. A free placefile resource is at http://www.redteamwx.com/grlevelx.html.
While very useful for those on a budget, you should remember that for free, don’t expect complete reliability or accuracy, and any recourse is minimal to none until the problem is fixed by the placefile creators. The same can be said with other free sources of data, including Iowa State and the NWS.
GREarth does not use placefiles, nor are there plans to introduce placefiles to that software.