Bear in mind that I was working in the industrial controls systems business when I put together my system so I was able to find surplus industrial grade materials to use.
The thermocouple type that I used is a Type K (Chromel/Alumel) thermocouple in a 1/8" diameter 304 stainless sheath with mineral insulation inside the sheath (called a Megopak assembly by Honeywell, my former employer). The element is about 12" long and has a quick-connect plug mounted on the end where I connect a cord to it. The other end of the cord goes to an instrument that can measure temperature with a Type K thermocouple.
The thermocouple is mounted in a way that puts the tip (where the temperature is actually measured) slightly above the lead furnace's discharge valve. With the sensor mounted this way, I know the actual temperature of the lead that will be discharged into my mold.
This type of thermocouple assembly is sold by several manufacturers and readily available through vendors like
Omega. Call them and they can easily help spec a sensor for you.
Of course, you will need an instrument that can measure temperature with a thermocouple. These days there are several electronics multimeters (which typically are used to measure volts/amps/ohms) which have the ability to measure temperature using a (typically Type K) thermocouple.