I think you just want any kind of strong tubing that fits firmly on the pointy end of your trekking pole.
If you are making something short, then the strength of the tubing material isn't a big deal, but as you head toward something longer and longer, the strength will become more of an issue. Some backpacking shops will sell a metal ferrule that is used to split a broken tent pole. You can use one of those, but they are only about three inches long.
I needed to jack up a pole one time, but I needed it to go only a half-inch. I found a plastic toothpaste cap that worked perfectly, except that it was green, so it kept getting lost on the ground. I painted it with glow-in-the-dark paint, and then I know right where it is.