I buy cd's, usually second hand for a few pence off amazon. just bought a load today actually.
I then rip them to my server as FLAC using dbpoweramp with acurate rip (similar to exact audio copy but better - repairs bad frames by using other people's accurate rips) Google play music reads the folder that they are stored in and I listen to my music through that at work (using radiant player on OSX as a ui for it)
my networked amps at home read the music on the server and stream the flacs through my hi-fi, or through winamp on PC.
I wouldn't buy MP3s, and I wouldn't buy individual songs. I would buy FLAC or WAV if it was availible though; but sadly isn't except from places like bowers and wilkins, linn tc. it's usually much cheaper and better to buy the CD and rip it!