Found this cool site for buying, creating and selling ringtones named Myxer.com. You can create and offer your custom ringtones for free using their free account or pay $29.99 for the year and sell your ringtones at any price you want! You can also place widgets like this one on your websites to help promote your cool custom ringtones.
So how do you make a ringtone from mp3? Well you can use Sony Acid to create ringtones using loops that you buy online or find for free on loop sites. I hope to provide some info on those sites and the best ones for ringtone making soon. In the meantime, here is a video on how to use Sony Acid to create your ringtones.