I am fairly certain that it is random, but your chances of getting one increases if you kill bosses like warrior or terra. --- PSN: KillingLegacy Gamertag: Killing Legacy "Be polite to others, and do not act in a way that you would not act in person."
I didn't farm Bunker for too long before he gave up his stuff that I wanted but if I'm remembering correctly he never dropped much in the way of relics. Terra is where I found my first Tediore relic. It wasn't the 30/70 but it was like 20/60 so it was close. I would definitely farm Terra. Unless you're incline to do BNK-3R. He has better Orange loot. IMO.