Journeyman:
Thermocouples aren't too bad as long as you aren't planning on building a low cost pyrometer for cast iron. A Type K thermocouple has a maximum temperature of 2300°F (1260°C) and a maximum continuous temperature of 2012°F (1100°C). This will allow you to use this pyrometer for most if not all of the non-ferrous metals. You can get a Type K thermocouple with a 6" SS sheath and 6' of armored cable pig-tail leads from Automation Direct for about $35 plus shipping. You might even be able to do better on Flea-Bay if you know exactly what you're looking for and can find the wheat amongst the chaff.
If you want to do cast iron you're going to be looking for either Type S (Platinum) or I think it's Type B (Platinum-Rhodium) thermocouples. To get one of those you'll have to go to Omega, or their equivalent, and order one - but be ready to pay for the privilege.
Don