If anyone is still looking for where to fish them, I found a location that is best suited for dedicated fishing alts. I fish for relics on the "Shipwrecked Ancient Relic Transport Vessel" in Margoria. I get about 3-4 in a night and about 20 relics. Note that I discard all fish, and my mastery is 1200. I have an alt in Port Ratt to manage the repair and storing of the relics using maids.