Use Healthcare.gov for all states except:
California: Covered California
Colorado: ConnectforHealthColorado
Connecticut: access health CT
District of Columbia: DC Health Link
Idaho: Your Health Idaho
Maryland: Maryland Health Connection
Massachusetts: Massachusetts Health Connector
Minnesota: MNSure
Mississippi: Healthcare.gov; One Mississippi for Small business employers and employees interested in SHOP coverage
New Mexico: healthcare.gov; beWellnm; https://shop.nmhix.com/hix/
New York: NY State of Health
Rhode Island: HealthSource RI
Utah: Healthcare.gov; AvenueH for small businesses
Vermont: Vermont Health Connect
Washington State: Washington Healthplanfinder; Washington Health Benefit Exchange
See also https://www.healthcare.gov/marketplace-in-your-state/