Seems you have found the
Faerie Quest Help! thread. As satiran wrote, you can buy it from these shops:
fairy_gypsy123 Healing Koi 1 950 NP
franciscobsbh Healing Koi 1 990 NP
jjs_babygirl Healing Koi 1 995 NP
cfbyd Healing Koi 1 995 NP
To get it, type in the username of the shopowner (i.e. fairy_gypsy123 , franciscobsbh , etc.) into the search bar at the bottom of most Neopets pages. Click on the username on the Results page. Find their shop (a link on their lookup will be on the right side) and click on the link. Go and find the item Healing Koi, and buy it. Then go to Faerieland, go to Faerie City, and click on Faerie Quests. Then complete the quest, and she will grant a stat boost to one of your pets (i.e. increase strength, defence, movement, Hitpoints, Level, etc.)