A site that I suggest my clients use is this one. Very user friendly and you can search by address, zip code etc.
Keep in mind, however, that what constitutes a crime, which labels someone as a sex offender, varies from state to state. In some states an 18 year old who has a consensual relationship with his 16 year old girlfriend can be labeled as a sex offender because she is under age. In other states, that is not the case. So, it helps to know state laws before assuming that one state has fewer than another. It can be based on something as simple as how their laws are applied.
I hope this helps!