A Google Map with all St. Louis are fields can be found here.
Forest Park Ultimate Fields
Larger than New York's Central Park, St. Louis' Forest Park is a true gem of the city. Most Ultimate pickup games and leagues are played on the Langenberg Fields just south of the Dwight Davis Tennis Center (near this Visitors' Center). The pin on the above map is in the right spot. Parking along Lagoon Dr. is an option as is parking in the Muny lot just East of the fields.