How do I accurately measure the B value of an NTC thermistor?
If you can wrap the thermistor in a waterproof heatproof enclosure (say, copper pipe soldered shut on one end) then an easy way to produce known temperatures is to use water's phase changes.

Boiling water is at 100 °C (with correction for your local air pressure based on altitude).
A mixture of water and ice (neither all melted nor all solid) is at 0 °C (after the water, if added separately, has cooled down; better but slower to let the ice melt to make the water).