Where can I rent some old cop cars?

I'd like to get a couple of late-70's vintage police cruisers for a movie scene I want to shoot. Don't worry, it's not a smash-fest. I just need some set dressing. I'd prefer Plymouth Furys or Dodge Monacos, but a Chevy Impala could work if it's Michigan State Police blue. Also, I'm looking for an ambulance from that same period. Links to any companies, or collector groups, with such vehicles available for use will be greatly appreciated. Thanks for the help.