A great way to find which phone carriers are available in your area is to find out which carriers are licensed in your area. WirelessAdvisor provides data on carrier licensing based on ZIP Code or city name and provides links to special deals available from those carriers.
While a carrier might be licensed in your area, that is no guarantee of high quality service. Even if the carrier provides high speeds in some parts of your area, that may not apply to you. Checking to see if there is a local store run by a carrier can be a good indicator of that carrier providing good service in your area.
Talking to people living in your area is also a very good way to figure out which carriers work best in your area. People you meet at work, friends and people on Internet forums based in your area can be excellent resources to find candid answers about carriers. As fellow users of those carriers, these people know the upsides and downsides of each, such as whether they provide good cell service in specific areas or whether they provide good customer service if you ever have issues with your plan or your phone.