These shut off valves are usually sold as part of an LP detector and are available from LP dealers or numerous online sources. Try a Google search for "LP shut off valve" (without the quotes).
But are you sure you need a new valve? It seems more likely your LP detector is triggering the valve because it thinks it has detected propane. If you are convinced this is a false alarm, you may need a new detector rather than a new valve. The sensor can get contaminated by things like hair spray or household chemicals, especially spray types.
Low voltage at the solenoid might also be the culprit.
Former owner of 2004 American Tradition
Home is in the Ocala Nat'l Forest near Ocala, FL