Custom exhaust

I visited a local company, to buy some 3" tube, that specialises in everything tubing and asked what is needed to bend a custom exhaust and they suggested two methods one of which could be considered as being more accurate: 1. Bend a piece of wire for the whole length of exhaust required and their CNC machines will copy it. 2. Buy a flexible piece of exhaust tubing of the right length and diameter (this is the tubing that is seen mainly on lorries that can be bent by hand and has a spiral design) and bend it exactly as needed. Then fill this tube with expanding foam (the foam that is used for building work) so it will hold its shape. Again this company's machine will copy it exactly in any tubing material required.