Hello alvin, You can buy Bartali Sailboat from Proix Tavern Owner in the Velia inn for 10m silvers or you can get it in a questline "The great Expedition Oquilla's eye".

You can also craft epheria sailboat/frigate at the workshop in Port Epheria. This method requires a lot of workers. The alternative is to upgrade the Bartali quest sailboat mentioned above, this requires some additional resources. You can also try to purchase the Ship License directly from the Central Market.