If you are looking for a custom hand fabricated SS exhaust then
MP Fab will do a fantastic job as Goolsby has stated. I've talked to Walker and have seen his fab work on a few occasions and it is top notch.
If instead you are just looking for custom bent exhaust then you might check with Clanton Muffler in Madison, AL. While he is a "regular" exhaust shop, he is pretty popular among the car crowd for doing good custom exhaust work. Not a lot of places want to mess with them but he is recommended often. I dont *think* he has a mandrel bender though.