Anything can be used. You can buy Jagwire liner, Nokon liner, Alligator liner etc, but any liner will do. You could also use heat shrink tubing if you wanted or something like the liner that is used in 3D printers. Anything that is light and flexible and slightly larger diameter than your inner cables, obviously.

Here in England the majority of stuff is a lot more expensive than in the States and we don't have the plethora of choice like you guys do. It makes people like me do DIY jobs on things like this Alligator i-link gear or brake cable set is around $80-90 USD here, for each set! I'm not gonna pay that.