Someone told me once about attaching a string/thin wire to the end of the housing BEFORE you pull it out of the frame. This is supposedly going to allow you to remove the housing and old cable, while at the same time pulling back through your "guide wire." Then, you do the opposite for reinstalling the new housing. I managed to replace an internal routed shifter line on a friends POS department store bike once, and I got lucky enough that just fishing it around for a few minutes yielded success. If I had known this trick that may or may not work ahead of time, I may have been inclined to try it. I don't know how you attach the guide string/wire to the housing that would allow it to pass through the frame hole, but I imagine it depends entirely on the size of the entry/exit holes. Thoughts?

[EDIT]
I seem to have misguided myself... In regards to the tube you are describing, I would check out the local hardware store in the section where they sell lengths of chain, cable, wire, beer bong tubing, etc. I see what you were asking now.