Tig rods can come in a variety of packages, some in round carboard or plastic tubes, some square/regtangle plastic or cardboard tubes. I think what you have seen may be waste pipe with screw ends fitted, you should be able to buy a length of waste pipe and the ends along with the solvent to attach the ends, probably get at screwfix or similar.
Yeah I can do that, will knock a couple up when I can so I have a reference for when I build the cart. What length for these would you reckon to be sensible?