You could book directly with the airline, steamship company, railroad or hotel. But most people preferred to use a travel agency, which would handle all details of your trip or vacation and deliver the tickets directly to you.

Some people, particularly fugitives from the law, would go to the docks and try to book passage on a cargo ship or other commercial vessel--cash on the barrelhead, no questions asked. If I remember correctly, that's how the novel says Michael got to Sicily after killing Sol and Mac.