Ok, I'll take a bite at this. Since Supernatural car is a 4 door it is not SS. SS are only coupes and convertibles.
Since the show aired these cars became more popular and people are asking ridiculous amount of money for them. 2 years ago my buddy couldn't sell his 4 door that was in great shape for $3500 now people are asking $10, 15 or even 20k for them.
There is quite a few of them for sale now days. Have you checked you local Craigslist?
Here is one in Topeka Kansas Topeka 4 door way overpriced
however I think you are better off buying a coupe. In a few years everybody is gonna forget about the show and the 4 door will be............just another 4 door and you won't be able to give it away let alone make even half of your money back. JMO