When we came back from Roatan to San Pedro, the flight to Belize was canceled because of a strike at La Ceiba airport. In short, take into account some kind of delay when planning your trip. American Airlines has a daily flight from Miami Continental has a daily flight from Houston From Europe, it's fairly simple. Iberia has 1 flight a day via Madrid with a stop in Miami. Otherwise you can of course also fly with American or Continental.
Price is usually a bit more expensive then a similar flight to Guatemala city.
The smaller ones are Trujillo, Utila and Guanaja. Tela has a small airport, but there are no commercial flights available there. Taca has international flights and works together with Islena . Taca has a website , but it doesn't have any schedules or prices. Sosa Airlines and Rollins air only have domestic flights.