When searching in library catalogs for materials related to dissident thought and movements, there are a number of terms you can use that address different aspects of the broader subject. Using any one of the following in combination with other subject terms like "United States," "History," "Sources," "Case studies," and "20th century" can usually yield good results:
Academic freedom
Aliens / Immigrants
Anti-Americanism
Anti-communist movements
Anti-environmentalism
Civil Rights
Civil rights movements
Communism
Dissenters
Environmental justice
Environmental protection
Environmentalists
Extremists
Feminism
Freedom of speech
Freedom of the press
Fundamentalism
Gay activists
Gay liberation movement
Gay rights
Gay-straight alliances
Green movement
Immigrants - Political activity
Internal security
Labor
Labor movement
Left-wing extremists
Minorities - Political activity
National security
Nativism
New Left
Political activists
Political crimes and offenses
Political participation
Political persecution
Political prisoners
Poor - Political activity
Protest movements
Radicalism
Right-wing extremists
Sedition
Sexual minorities
Social action
Social movements
Social change
Social justice
Student movements
Students - Political activity
Subversive activities
Tea Party movement
Tea Party Patriots
Welfare rights movement
White supremacy movements
Wise Use movement
Women in the labor movement
Women's rights
Working class
Youth - Political activity
Youth movements