You can buy cryotek from lowes and home depot wherever you find boiler and home winterizing sections in the store. Mix with water in a factory recommended ratio and it will get you by in a pinch for most rotovap operations.
Like all glycol mixtures its only good down to certain temps. You DO NOT want it to turn to sludge. That higher viscosity cold sludge is going to reduce your heat transfer considerably, if you have this issue then bump the temp up a bit. A chiller running smoothly at 4c is going to be better than a sludgy one at -10. Heat transfer is not just about the fluids ability to absorb energy but also the systems ability to cycle coolant through the lines and heat exchanger in the chiller efficiently, otherwise your condenser in the rotovap its self will loose some heat exchange capacity.