If you are going to powder coat it black, just buy 7" pipe from a truck supply house for cheap and cut it at an angle with a band saw. These are that way and look nice...minus the angle cut.
We didn't have them clamped up and leveled yet, so they are not completely vertical in the pics.
Black Cloud has raw stacks ranging in the mid 100's but that's about the only ones I could find. I was looking for one that was already powdercoated or teflon coated