If you haven't tried them already, since they're still in business it may be worth contacting them first and see if they happen to have any of the correct era ones still around?.
Make sure to ask about old files/documents too!.
http://www.selmanchevrolet.com/
...Howard Selman founded Selman Chevrolet in Orange in 1952. This month the company celebrates 55 years of dedicated service to the city of Orange. The current President, Bill Selman and his family are dedicated to continuing the proud tradition of excellence in pricing, service and repair that Howard started.
Selman Chevrolet in Orange has 262 employees; some having been with the company for more than 30 years. The employees are treated like family and enjoy a wonderful company environment. They work hard and are well compensated for their dedication to the company...
~ Pete