My two five series both have the original muffler. 11 year on the original I must say it is very durable. I would think all major mail order outfits have them. Check out AutoHauz.
Be fore warned, you are in for a lot lot work if you DIY. The muffler connection to the cat will give you fits. My experience is the replacement muffler lasts about four years, ansa or bosal.