Pork Chop BMX is my favorite source for the clear plastic covered brake cable housing that are identical to what many bike manufacturers like Peugeot, used on their sport and race bikes in the 80's.
Very cheap prices too, plus I think they sell they brake cables and casings in bulk too.
Check them out at eBay.....
Been buying the cable/casings from them for some years now with no problems and shipping is really quick, plus they throw in nice standard size ferrules and crimp on cable end caps with every set they sell.