How do you get medieval bricks?

Same way you get modern ones, but without the aid of any powered tools or machinery.

Clay bricks are a certain quality of mud shaped into a mold and then dried and fired in a kiln.

Stone bricks are cut and shaped from chunks of rock using tools.

Concrete bricks are formed of an aggregate of special types of sand or gravel, and limestone powder. (Concrete has been around since the time of the Romans, who used it a lot.)