There are several sources--but they are not free. WestLaw is the one I am most familiar with. There is another higher end program that lawyers use. Usually you have to subscribe to each state that you want to check.
Most states have their revised statutes on line, but they are not annotated and do not give the case law that was used to define the statutory law. There are also some services that track legislation in each state and have references to revised statutes and case law, but again, their are some hefty fees associated with that.
Most library systems, at least in larger communities, will have a set of annotated revised statutes available for use in the library at no charge. These are usually in the branches that are near the courthourse and where law offices seems to cluster together.