I would not recommend buying music from iTunes. Most iTunes music contains (or used to contain) DRM or Digital Rights Management which means the file itself contains code limiting the access you have. In laymens terms, if you buy songs with DRM then you'll be limited on what you can do with it. In the case of iTunes it limits the number of times you can burn a song to CD and prevents you from playing it in other media players.
The most popular place to buy DRM-free music is Amazon.com
Here is a link:
http://www.amazon.com/MP3-Music-Download/b?ie=UTF8&node=163856011
You can download songs for about $1 each and full albums for around $10. The prices vary based on the music's popularity or current deals they are running.