Final Fantasy I & II: Dawn of Souls

If you talking about the one that belongs in Gaia, you don't find it in a city.
Go East from Gaia until you find the other northern continent to the side of the "Hawk" continent Gaia is on (there's only one town on that continent). Find the biggest desert there, it should be easily found. You'll notice a piece of desert that is separated from the rest of the desert, go there and walk around that area, you should run into a tent (aka: the caravan).
Buy the bottled fairy there for 40000 Gil.

Also, once you buy the fairy -- go back and talk to the guy and you can buy the different tonics there.

www.gamefaqs.com/portable/gbadvance/file/920240/33606

Its on the map,its in the desert west of Onrac to the left of the trees just walk around it and you will enter the caravan;after you buy the bottle,take it back to Gaia and then visit the pond.

Well their is a desert near the mirage tower go their and you will have to buy it for 1,000,00 so have fun beating the game