You can usually sit in some older ones at air shows, or air museums. Sometimes they'll let you sit in new ones too! I got to sit in an FA-18 when I was a kid at an airshow, but they had all the gauges blacked out so you couldn't see classified parts of the jet. You can sit in the cockpit of an SR-71 Blackbird at the Boeing air museum in Renton, WA, but it is detached from the rest of the plane.