Those things are fragile enough that there are very, very few of them just laying around. There is a vendor on eBay selling an approximate reproduction; fine for a driver but not for what I would call a "restoration". Most likely, you'll need to find a broken original and have it rebuilt.
However, as a temporary substitute, almost any 2" mechanical water temperature gauge will work.
I'm actually using the temp gauge from an early TR4, which is electric rather than mechanical. It looks very nearly identical to the original, but requires some other modifications like a voltage stabilizer, correct sender and a wire to the sender.