[Eru]

There is a quest giver in Ocllo that tells you what to do, but the actual pole is bought from a vendor stone next to the NPC. Basically, you buy stuff for fishing with sea serpent scales. The pole is about 3000 scales, which is a bit of a pain to get, but it goes faster than you think. You can buy bait from the vendor stone or from turning in 'big fish' to the 'Fishing Contest Officials' placed in various cities.

Also, since deep sea poles aren't linked to your account or character, they can be bought and sold from other players.