Princess Auto.
They sale price a digital multimeter that uses a Type K thermocouple, for $10 each, about 3 times a year.
Find some stainless tube. Run the thermocouple wire through it, crimp the end over the thermocouple wire with a bench vise, or a hammer (carefully). Take it to someone that has a TIG welder, and get the crimped end welded solid. Just a fast fusion weld across the end of the crimp.
Bend as required.
Make up a chart showing Celsius to Fahrenheit (the meter reads degrees C).
In on the pretty cheap.
I think C Tire sells a thermocouple type meter as well, but not as inexpensive.
Lee, Lyman, both sell a casting thermometer. Check their sites for the product number.
Cheers
Trev