Cat Quest

You get weapons and armor in 3 different ways: Quests, dungeons & purchasing them. For the more common ones, you will get the majority of them by doing side quests & opening chests in the dungeons but can also buy a chance at random ones from Kit Cat's for 50 gold each. The rarer ones are going to be locked in the larger chests in the dungeons or purchased for a random chance for 5,000 gold, with a few coming from quests. You will have to do a combination of all of them in order to get all 66 of the armor and weapons required for the trophy. A good place to grind out gold to go after the purchased ones is to go to the Founder's Cave on Founder's Island (just south of Felingard) as you get over 1,000 gold each time you clear it from the enemies in there. Just keep finishing off the side quests, completing the dungeons and buying weapons/armor until you hit 66 items at which time the trophy will pop.

Note: There is a possibility of some items being glitched and not appearing in the chests they are suppose to (Samewrai Helmet seems to be biggest one). If you finish the game, all the side quests & get all the chests in the dungeons and are still missing some items, you will need to farm gold and buy chests from Kit Kat's to try to get them.