> A heavy but durable solution I used once is flexible copper tubing
> available at most hardware stores. It is used a gas/oil lines in houses.
> It tends to be bigger than you need, but it does work.

Low pressure hydraulic steering systems for small boats(less than 35 feet)use teflon or nylon tubing,fairly stiff,and small enuf bore to carry a steering cable. Can't be too expensive- you won't need more than 25 or so feet and it is self lubricating.