It is not plentiful in daily life. It is formed by the combination of organic materials with chlorine, such as in bleaching of paper. It requires a temperature of at least a few hundred degrees, or presence of a catalyst. It may be generated by incineration of waste containing chlorine. PVC, especially construction waste in trash, is a common source.
Copper is catalytic, so burning electric wire is a prime source.
This shows why backyard burning is such a dangerous and foolish thing to do. Backyard burning of PVC also gives off Hydrogen Chlorine, which forms Hydrochloric Acid in the lungs. This can result in serious lung damage and Pneumonia. We do not expect to find dioxin in PVC products at room temperature, although in fact we do occasionally find traces of it in products manufactured with less than ideal quality control. VCM is also a potent neurotoxin and carcinogen.