To get these, you need to trade specific fishes to an Imperial Fishing Delivery NPC.

Imperial Fishing Delivery NPCs can be found in Velia, Glish, Epheria Port, Splashing Point and Valencia City. They will take most blue/yellow fish you have at 250% trade bonus and occasionally give you Shiny Golden Seal – [Imperial Fishing] as well.

* There is no distance bonus. The trade window may include a higher price since it factor in the distance bonus but you will only get paid the 250% trade value with no distance bonus.
* You cannot bargain with the trader.

There is no distance bonus but you still need to have nodes connected or you will only get 30%. The freshness of the fish won’t matter for the silver value, as long the fish is not expired. Each trade also have a limit on how much of each fish (usually 30-40) they will take and this limit reset every few hours (assumption is randomly every 3-4 hrs). This limit is shared with other players so you swap channel or NPC if you encounter a NPC that no longer takes your fish.

Source: [link]

Is this obtainable in any way now?