WALK-IN:
Convenient care clinics (CCCs) are health care clinics located in retail stores, supermarkets and pharmacies that treat routine family illnesses and provide preventative health care services. They are sometimes referred to as “retail-based clinics” or "walk-in medical clinics." CCCs are usually staffed by nurse practitioners (NPs) or physician assistants (PAs). Some CCCs, however, are staffed by physicians.
FREE(アメリカの説明ですがほぼ同じだと思います):
A free clinic is a medical facility offering community healthcare on a free or very low-cost basis in countries with marginal or no universal health care. Care is generally provided in these clinics to persons who have lower or limited income and no health insurance, including persons who are not eligible for US Medicaid or Medicare programs. In the US, almost all free clinics provide care for acute, non-emergent conditions. Many also provide a full range of primary care (including preventive care) and care for chronic conditions. Some free clinics include licensed pharmacies and dental services.