A few years late, but I'm just adding my two cents because I stumbled upon this post and had the same question.
While not definitive, it seems like my version of Google Earth (v184.108.40.20676) lists the source on the bottom portion of the screen. Here's an example over the Western U.S. -- you can see it's a combination of Landast/Copernicus data and some U.S. ...
You can try the USGS data portal:
I have acquired data below 4 m from the site. You use their search tool and select what you are interested in and apply the proper filters.
You can also use the national map to source other data from the US