In embolus-type strokes, what is the usual source?

Heart or neck artery. One common place emboli come from are the heart if you have a certain type of arrhythmia or if you have other types of heart defects. The other place is from the main artery in your neck called the carotid artery. Plaque can dislodge from here into your brain. .
Multiple. Emboli can come from the heart (blood clots as result of atrial fibrillation for instance), or from the aorta and carotid arteries (usually calcified or non calcified cholesterol plaques).
Heart. Most strokes are associated with hardening of the arteries and formation of clots in the carotid artery in the neck. These produce small emboli to the brain, but when we speak of major embolic strokes, we generally think of origin in the heart, either from atrial fibrillation (abnormal contractions of the top of the heart), heart valves, or loss of movement of the heart wall.