old type thermostat ?

Hello Rob. Wow it's a long time since I've seen one of those. It looks just like the one Hoover used to use in the 70s. I would think it extremely unlikely that anyone is still selling them as they stopped using them many decades ago. However, they were extremely reliable and I don't ever remember having to replace one. How sure are you that this one is faulty? If I remember correctly they can be tested by placing it in some water so that the water just covers the metal part at the bottom.

The middle connection is the common which is normally fed a live feed. There are just three bimetal parts reacting to different temperatures. So if you connect continuity tester to the middle connection there should be no continuity between that and each of the outer connections.

But at say 30 or 40° you should get a connection between the middle connector and one of the outside ones (probably number one). Then at about 60° you should get a connection on one of the others, and at about 90° you should get a connection on the third one.

Then as it cools down it should return to no continuity.