As you go through the game, make sure to search for treasure chest as these almost always contain armour pieces. Most of the quests you complete will reward you with equipment to use also. However, to get the most equipment pieces, make sure you talk to everyone in an area after you finish the main storyline there. Most people will thank you for helping and gift you with another equipment piece.
There are over 80 pieces in the game total so there's plenty of room for error.