The thermocouple does not in any way come into contact with the elements.
I believe they will last indefinitely if you do not break them. They are easy to break because they are small and delicate. I broke one of mine and my friend Terry Fallon fixed it.
The caustic nature of the crystalline atmosphere does not touch anything but the porcelain sheath.
I believe Omega is one company that makes them and Cleveland Electric is another. And most kiln manufacturers probably stock them too.