With 87,000 employees, the IRS is the world's largest law enforcement agency. Their job is to collect taxes and go after those who don't pay their taxes.
Most states have a state police force of some type. They may be called - the State Police - the Highway Patrol - State Troopers - etc. Some states have specialized law enforcement bodies (i.e.- Texas...
The United States Border Patrol has approximately 20,000 patrol agents, and is second in size only to the New York Police Department, with about 38,000 sworn officers.