The most common source of pulmonary embolism is-
Pulmonary embolism

Pulmonary embolism is a blockage of pulmonary artery or one of its branches.
Pulmonary embolism is an extremely common and highly lethal condition that is a leading cause of death in all age groups.

Etiology

In more than 95% of instances, venous emboli originating from deep leg vein thrombi above the level of knee i.e. in popliteal, femoral or iliac veins.
Although the most common site of deep vein thrombosis is calf veins, calf veins thrombosis is itself associated with a low risk of embolism, but when the thrombosis progresses proximally to involve the larger veins above the leg then the risk becomes very high.
So,  Most common site of DVT → Calf veins

Most common source of pulmonary embolism → DVT of proximal veins of lower limb i.e., popliteal, femoral or iliac veins.
Pathophysiology of pulmonary embolism

If the size of embolus is large, it may be impacted at the bifurcation of main pulmonary artery → Saddle embolus, or may be passed out into the smaller, branching arterioles.
More commonly, there are multiple emboli, or a large embolus may be fragmented into many smaller emboli which are then impacted in a number of vessels, particularly affecting the lower lobes of lungs.
Rarely, Paradoxical embolism may occur by passage of an embolus from the right side of the heart into the left heart through ASD or VSD. In this way, pulmonary emboli may reach systemic circulation.
Amniotic fluid embolism
Renal artery embolism
Large veins of lower limb
Cardiothoracic surgery