If your cell phone can play MP3s, then you can send it to them.
But you'll have to reduce the size. Compress it to about 64kbps mono (anything else is a waste on a phone), and crop the time down to less than 30 seconds. If the file size is over 200KB or so, it's not going to be able to download it. (Some phones allow even less, it all depends)
Almost all ringtone and screensaver sites are either massively expensive ridiculous pay sites, or virus-filled horrors. I'm sorry, but I refuse to pay 95 cents for a ringtone of a popular song, when I could buy the actual song off of iTunes and convert it myself.