Maybe try a local exhaust manufacturer like PowerFlow. If not you will need to source the pipe from a metal specialist.
Not sure how they will ask for sizes but to me sch 5 is the thickness (about 2mm a wall)
Im having mine replaced aswell.
Try your nearest Truck parts Factors they can source "Dinex" exhaust products which include every size of pipe, bend, flexi or silencer in stainless or mild steel, you can also get flanged pipe / bends that slip together.