These are currently in the transmog tab with vendor as source. Eventhough WoWhead says that they are not in game and in 2018 they have been used as an april fools joke.
Some people have speculated that they are from the trading post but the upcoming trading post selection doesn’t offer them and none of the other trading post items are in the transmog tabs before you buy them.
This makes me believe that it is available trough other means.
Someone in the Secret finding discord asked a Gm and eventhough they did not get an answer as to where to find it, they did more or less confirm that the item is in game.
-The Gilnean Quarter master does not sell the item.
-None of the vendors in the Gilneas starting zone sell the item.
-No vendors inside, nor around Shadowfang Keep.