I bought my 4" straight pipe from a local truck shop/cummins dealer. I had to buy a 10 foot section though. It all worked out in the end, I used like 6 for the straight pipe, then later added stacks, so the remaining 4 foot section worked perfectly.
They will also have your muffler clamps or band clamps, which ever you prefer. Its the same place I buy all my Fleetguard filters.