To get started quickly, I recommend the Gale Force Nine islands they made for the game (affiliate link). They’re a similar size and give good bang for buck.
Beyond that it’s a wild west of customizing your own islands and terrain. A bunch of mine are on this page, with many other prettier examples found throughout the community (such as on BGG).
I look forward to making some terrain for Pirates. I’ve made a few islands before using painted pink foam and various textured flocking, but they were for Dystopian Wars so the scale was much finer. This scale should prove more of a challenge since more detail needs to be added.