Problem is, you need not just the tube but the ether inside.
One way is to buy an inexpensive mechanical temp gauge, and graft it's tube & bulb to your gauge. Here's a link to one article on the subject; there are more posted in other threads on this forum.
https://www.ply33.com/Repair/tempgauge
Another way is to send it off to Mo-Ma or Nisongers to be rebuilt.
Converting to a TR4 gauge is pretty easy. You'll need the gauge itself, a voltage "stabilizer" and the sending unit. Mount the stabilizer somewhere it gets a good ground (original was to the back of the speedo, but somewhere like the heater bracket or firewall near the fuse block would be easier, IMO). Run a wire from the sender to one terminal of the gauge, then another wire from the other gauge terminal to the "I" terminal on the stabilizer. Then a green wire from the "B" terminal of the stabilizer to the original green circuit somewhere, like on the fuel gauge or fuse block. That's all there is to it.