Check out these guys. They supplied the kit that is on my truck. Look for a conversion center close to you. The laws may be different as I am in Canada and you are in USA (EPA and such). Kits here don't have to be emission certified but they do have to meet safety standards. My province doesn't require emission tests on new vehicles until they are 7 years old. And then it can be tested on either CNG or gasoline.
Prices are going vary depending on what tanks you go with, local laws, federal laws, installation charges, kit prices etc. I went with a used, recertified tank and traded in my old tank from my 2000 Dodge Dakota.