I want to get a louder sound from my 2005 bmw 645ci. Is there a muffler that can do this for me or would I need a full exhaust system. If so how do I go about getting it. I would prefer quad exhaust to my dual exhaust if possible. Any websites of choices or installers in the orange county CA area are much appreciated.

Yes theres SO many options out there for custom exaust.
Just find one in your price range and take off that cadillac convertor if its legal in your area, because that will lower your emmissions.
basically you just have to match up the pipe size and hook it up and you will get easy horsepower and even more engine music