Most gas stations sell it, like Pep Boys, Autozone, and Chevron.
Antifreeze is a liquid that is added to the water in an engine's cooling system in order to lower the freezing point. This means that the water will not freeze under normal cold weather conditions. If that water did freeze you'd be in a spot of bother there - it'd cause some serious damage to the radiator and other components, which would cost an arm and a leg to fix.
As well as preventing water from freezing up, antifreeze raises the boiling point of engine coolant to prevent overheating. The stuff also protects your engine from corrosion, aids heat transfer, and prevents scale from building up internally.
Find out which antifreeze is right for your vehicle at Chiltondiy.com.