High Test Eternium Fishing Line

The High Test Eternium Fishing Line is an enchanted fishing line that will add +5 fishing to a fishing pole. This effect will not stack with other enchantments on the item.

Source
This item is a reward from winning the Stranglethorn Fishing Extravaganza by turning in the quests Grand Prize, Second Place, or Third Place.

It is also a quest reward from the quest Rare Fish - Dezian Queenfish, which is turned in to the next to Riggle Bassbait.

Finally, it is also occasionally found in the Bag of Fishing Treasures (Dalaran) from the daily fishing quests given by Marcia Chase in Dalaran.