Originally Posted by rm -rf

Tubing
Contact your local bike shop. The mechanics should have a roll of the tubing, and should be able to give you or sell you a piece. Get some crimp ends at the same time.
I've never heard of an official name for the stuff. Tell them it's the temporary thin tubing for fishing new cables through the frame...
See my post in a previous thread here. There's a Park Tool photo (with blue gloves) that you could show them.

~~~

Even the cable sets will need to have the housing cut, and the cable shortened after attaching to the derailleur.

I like to get the housing the right length. New bikes usually have excess housing at the bars, since they need room to do adjustments for the buyer. I insert one end of the cable in the shifter, without an internal cable, and temporarily tape it to the bars (if it's an under the bar tape cable.) Then I can hold the other end at the frame stop and estimate the correct housing length. I cut it and decide if I need to shorten it a little more. Then smooth the cut end--I use a dremel with a grinding wheel.