You can use the port city coordinates shown in Storehouse to set sail. 

Tap on the coordinate and set sail, or use the Set Sail button right next to the coordinates.

■ How to set sail to port cities

- [Main screen] > [My Territory] > [Storehouse]

> [Tap on Trade Info] > [Choose Market Price Info tap] > [Choose the Set Sail button next to the port city you want]