Using liquid nitrogen to make a slush bath out of R-12 refrigerant fluid "Freon-12" -- difluoro-dichloromethane will give a stable temperature of -157 degreec C. From the time the nitrogen boils off, until all the frozen R-12 melts, the temperature will hold at -157.
Another option is to find a testing lab with an ultrafreezer to perform the inpact testing [Charpy?] for you. Your specimens are small and light. Airmailing them to a lab is fairly inexpensive.