Mechanical filter can be purchased ready to assemble as kits, or parts can be purchased separately and assembled. I have found that it is much cheaper to purchase a pump and replacement filter media and assemble them yourself. Mechanical filter provide some biological actions, but there design is mainly to provide removal of particles.
It is useful to route the exiting water to an aeration device such as a fountain head or water fall or statuary. Also zeolites or activated charcoal can be bagged and placed inside the mechanical filter to remove chemical wastes such as ammonia which is toxic to your fish and other non-airbreathers.