This is the only remaining Saturn V stack made of flyable parts. It was built for Apollo 18 which was scrubbed when the Apollo program was canceled. It is in Rocket Park at the Johnson Space Center in Houston, but owned and maintained by the Smithsonian. Though it may not be obvious, admittance to Rocket Park is free. You simply drive to the Space Center entrance and tell the guard on duty you are going to Rocket Park.
Visitor hours are something like 8am to 6pm.