Mine came from the guy that posted here ^ post #3. It was awhile ago, so I don't recall part number off hand. I'd have to do some diggin'. I'm sure Scot could help. I believe mine is 1 1/8" though
I had thoughts of getting the 4" also. But went with their 3" setup. I think it sounds fantastic at idle, and inside I have no complaints about how loud it is. I have no regrets.