Where can i download music like online legally?

I want to download songs to my iphone like "dancing in the water (wild boys)" by avicii. Preferably free but if it isn't possible to do that legally then i'm willing to pay.Ps. It isn't on itunes

Added (1). Is youtube to mp3 legal

Just do the youtube to mp3 converter…

http://www.youtube-mp3.org/

Rhapsody

DEFINITELY www.onlinevideoconverter.com

Downloading youtube is not legal.that's why they put a download button there.
Free is ilegal.

Amazon, Walmart are a couple more sites