I built a new loom for my '78 550K last spring using all new wire bought through a friend of mine who is a professional auto-electrician.
However, the last two looms I built for our competition cars (my usual evening time filler) were built using wire salvaged from nearly new insurance write off cars.
I don't know how it works on the US, but here in the UK, auto breakers generally don't consider a wrecked car's loom to be a value part, so you can pick up really good quality stuff for next to no outlay