How to become an ISP is a pretty involved question. Long story short I wouldn't recommend doing it unless you have a huge amount of capitol up front. Unless you become a reseller of another providers service. The profit margin will be small but if you can get enough subscribers you may be able to make it worthwhile.
ISP's buy their service from basically the next step up of internet providers. They are referred to as major internet backbone providers.
http://www.webopedia.com/TERM/B/backbone_provider.html
