Both destinations are served by Colombo (CMB). Since there's no flight route between them, the best way to travel is by road.
Coimbatore
Fly to Colombo (CMB), then travel by road to Coimbatore. Taxis, buses, and private cabs are available from the airport.
Attingal
Fly to Colombo (CMB), then travel by road to Attingal. Local transport options include taxis, auto-rickshaws, and state buses.