You can buy bulk, teflon lined, 5mm brake cable outer housing at your local bike store. Get some matching ferrules for the ends as well. If you don't have the proper parrot's beak cutters, use a file or grinding disc to make the cut ends square. I use VW type 2 throttle cables for the inner. They have a kinked end the works well and they are more than long enough. I got mine from
www.cip1.ca. Cheap and cheerful and you can also get very useful things like 11mm hex, 8mm thread, brass, deep hex nuts as well.