Q: I want to purchase music on itunes from a different country, Brazil. How do I do that?

You can't, you can only purchase from your own country's store - to be able to buy from another country's store you need a valid billing address in that country and be in that country when purchasing from it. You can try requesting that the music that you want be added to the US store, but ultimately it's the record company that will have to grant Apple a license to sell it there : http://www.apple.com/feedback/itunes.html

Is the music that you want available on other music purchase download sites that you have access to, or can you buy the CD and copy it via your computer's iTunes ?