The original brake pipes fitted at the factory are made from steel, over time they rust and deteriorate. During an MOT I'm pretty sure they inspect the pipes visually and measure the diameter of the pipes to figure out how much swelling has occured from the rust and ascertain if they are safe and road worthy. Replacement pipes are generally made from copper and are bent into shape to fit. Given the materials, shape and dimensions of brake pipes I don't think it's necessary to sell them as off the shelf parts, think of it as tubing that can be cut and bent into almost any shape and length, hence 'custom made' From what I've gathered the rear subframe and fuel tank has to be dropped in order to gain access to remove and fit new brake pipes. As a result the labour charges can be quite high. Hope that sums it up for you.