I can answer this!
(I can answer this because I was googling like crazy for the answer just two days ago.)
You can't find it. Google Play music puts it in a hidden folder you can't get at with a standard file manager. (I did read where it is possible to get into the folder if you root, but since I don't root, I didn't read the details on where to find it.)
The Combined Wisdom of the Internet on this seems to be that if you want the music to be available to other music players, you need to download it from Google Play to your PC, and then move it to your phone yourself.