The commonest cause of pulmonary embolism is -
Thrombosis of leg veins
Thrombosis of prostatic veins
IVC thrombosis
Thrombosis of internal pudendal artery
Thrombi in a deep vein of the leg accounts for more than 95% of cases of pulmonary embolism.
The most common source of emboli is DVT of femoropopliteal and iliac veins.
But remember that :

Most common site of DVT is calf veins
(Calf veins thrombosis are itself associated with a low risk for embolism, but when the thrombosis progresses proximally to involve the larger veins above knee, then the risk becomes very high.