Mississippi residents shop for ACA health plans through HealthCare.gov, the federal marketplace, since the state runs no exchange of its own.
Mississippi has not adopted Medicaid expansion under the ACA. This leaves a meaningful coverage gap, since the state's traditional Medicaid eligibility rules for adults are especially narrow, and many low-income working adults earn too little for marketplace subsidies to make coverage affordable but don't qualify for Medicaid under current rules.
Given how large the coverage gap can be in Mississippi, residents should check every possible subsidy carefully during enrollment, since even modest premium tax credits can make a real difference. It's also worth looking into community health centers and sliding-scale clinics as a supplement for people who remain uninsured while sorting out marketplace options.