K&S sells "antenna tubes" appx 48 inch clear plastic tube about the dia of Ny-Rod inner rod. It would work very well for the tube to run a pull-pull inside.

Sullivan sells a 30 ft coil of thin stranded steel cable type pushrod with tube... The outer dia of the nylon tube is smaller than the normal nyrod inner rod. (about the dia of a 4-40) this would nake a heavy duty pull-pull system (comes with a few pieces of threaded 2-56 brass tube to solder on the ends of the cable... ) I still have about 20 ft of my coil that I bought 3 years ago. I don't use it a lot but when I want it... there is no substitute. (great for sailplane spoiler and flying stab on T-tail operation)