If you are searching exclusively on the internet for specific Mp3's try these sites. Some only allow online streaming while others, like plorf, allow for direct download of the actual Mp3's. Great for when you are away from your personal music library.
New Music Exploration Websites. Each site has different approaches.
To search by similar artists try
New Music reviews
To search by Genre
To follow bloggers and songs in real time as people on the internet are listening and recommending check out.
Now a track of music