Fleet Commander

You can buy ships from any technician (the same place you go to upgrade your existing ships). Also there are a couple of quests where a ship is a reward; e.g. if you pick up a note speaking of a Hidden Outpost.

Otherwise, when walking around on a planet sometimes (often!) a ship will land nearby - some of these are crewed by enemies and can be taken over once all the enemies are killed. Usually, though, you'll need a higher Piloting skill to be able to take them over.