Where do I find the Google Drive icon?

Find the installation folder of Google Drive and the executable googledrivesync.exe (e.g. C:\Program Files (x86)\Google\Drive\googledrivesync.exe).

Right click -> Properties -> Change Icon

Since most Windows versions and Windows programs have moved or will move to 64-bit, this icon can now be found here. Take note, the icon displayed on the .exe is (probably) the one you're looking for, but that .exe contains a dozen or so icons if you choose it as a source, just like most do.

C:\Program Files\Google\Drive\googledrivesync.exe

If you happen to still be on a 32-bit system (doubtful), then as duDE said above, it will be:

C:\Program Files (x86)\Google\Drive\googledrivesync.exe