I did a little digging, and unsurprisingly it seems tungsten tube is not an easily sourced material. However, it got me thinking, what about a cheap tungsten carbide wedding band? granted, you are only going to get maybe 12mm of “nozzle”, but they are available for 10 bucks.
So here is my thought:
you first drill a slightly larger hole in a firebrick that will fit the nozzle, chisel out the material, then drill out the center with the size you want your nozzle to have. the ring rests on the lip, and maybe you cement it in there with some furnace cement. The whole job only costs 13 bucks, and you get yourself a nozzle tip that should handle 2800 degrees centigrade. The brick I would think would wear out first, and it can be replaced for a couple bucks. You are limited a little bit by the available sizes of wedding rings and hole saws, but if you look at the sizing chart, it looks like you can get between 17 and 24mm just with that particular ring.
Any thoughts on that nozzle idea?