Mp3 URLs can be found on a lot of media hosting sites on the net. You can use a search engine to look for mp3 urls or a site with your particular song.
Some sites out there that have urls AND allow you to upload your own music include mysplayer and mailboxdrive, but there are bound to be more, and if you find some, let us know.
Notice: Because of the music industry and their fight against piracy, it's up to those sites who post urls and other services to make sure that they are not in violation of any rules. Although it is becoming more and more common that sites are able to provide LEGAL music hosting services.