Where are calluses most commonly found?

Hands, feet. Calluses are a response to repeated pressure and rubbing and are a protective measure by the body. The hands and feet are the sites where these are common.
Hands and feet. Areas of pressure and friction.
On the skin. Not to put too fine a point on it but, they are usually found and formed between a bone and a hard place. Any skin trapped between an underlying bone and an externally applied pressure is apt to form a callus. That series of circumstances just occurs most of ten on the hands and feet.