Mild steel is the easiest to work with ( and more cost effective ) Stainless is a bit more $$$.
Try Builtwell, they have exhaust builder kits for HD's, nice selection of tube/bends. I used one of thier kits to start and then bought a collector and a few more bends from Columbia River Mandrel Bending. Also used Mid-Usa's torque enhancers ( bassicaly velocity stacks for your head pipes...increases velocity at low RPM to keep some semblance of low end torque due to the HUGE 1.750 pipes I used. )
Linky:
www.mandrel-bends.com/catalog/?gclid=CJuRzvjp-qQCFYL75wodkWsTzA
