Doug,
It depends on how particular you are about your engine bay. You can certainly go out and get an Interstate or Sears battery or some other reputable brand for a lot less, but many prefer a factory battery. Mind you, the factory battery costs far more, is probably an inferior product to a Diehard battery for the warranty alone, and bears no resemblance to the original factory battery from the 60s/70s.
The dealer is probably the best choice if you go the factory route because most parts distributors don't like to ship batteries.
Douglas Kim
New York, NY
280 SL #016220