Solid Gold Coin

Source
Solid Gold Coin can be found inside the Bag of Fishing Treasures (Dalaran) rewarded from completing Marcia Chase's daily fishing quests in Dalaran.

Additionally, players who utilize the Faction Change service with items in their bags that cannot be transferred to the other side (such as faction-specific quest-starting items) will find those items converted to Solid Gold Coins when they first log in after the faction change.