Amazon has a system similar to iTunes that allows you to download MP3s for around 99 cents each. The only difference is that you can often get free credits to Amazon and now is one of those examples.
You can get a $2 credit for putting in the promo code HITUMBLR (You don't need a Tumblr account). Just go to this link and put in the code. You need to use your credit before April 1st.
You can also get a $1 MP3 credit for buying certain kindle books. One of the books is free for Amazon prime members so if you "buy" the book, you will get a free credit. You need a kindle or to download a kindle app.
This is all on top of the $2 credit I got last week for Amazon student.