To get the Fisher King outfit, you need to get the Giant Worm by digging at a statue in the Eastern Pyramid of Aegis.
Once you have it, go to Ft. Eagle. Wear your Thief costume. Go through the forest, using Snatch on enemies and NPCs if you have enough EP. When you get to the area that has a bunch of slimes, use the northwest exit. You will get to a screen with some cows, and a guy who is fishing.
Give the worm to the guy who is fishing. Take the Fisher King outfit when it appears, but don't change into it yet. There is more that you can do with the Thief outfit here at Ft. Eagle. Read about it on the Bamboo Island page.