Just use the inner teflon/nylon tubing (usually black or grey in color) that comes in lined brake cable housing. Your local LBS might throw some out when they do brake servicing on bikes. The outside of the casings might be beat up, but you should be able to get some good legnths of the liner from them as they should be OK being protected by the steel outer casing material. They just pull/slip out easily.

Chombi