You buy one with the one drachma coin that you earn by cleaning graffiti from the Temple of Hades.
Once you pass the Olympus gate (late in the game), you will find the Wind Wagon run by Aeolus. Buy a windbag to travel up to the top of Mount Olympus, where you will battle Zeus.
You help a guy from Hades Realm to clean the grafitti on his temple. Then he gives you a drachama for a token of thanks.
The one-drachma coin for cleaning the Temple of Hades is used late in the game, when you travel to Mount Olympus. The drachma buys a "windbag" that can lift you up the mountain.
To find Aeolus all you have to do is get through the Mount Olympus gate (Hercules will help you) and then buy the windbag using one golden drachma from Aeolus on the trail to the top of the mountain, which is where you will battle Zeus (later).
When you go to Mount Olympus (near the game's end), you can use it to buy a windbag from the Aeolus Wind Wagon. This is the only way to get up the mountain.
You need a windbag to go up up and onto a rock. hen you go onto Zeus's statue and go through the pink mist.
You need a drachma to get the windbag on Mythology Island in Poptropica. If you removed the graffiti at Hades' temple but didn't receive a drachma, you might have missed interacting with all the characters in the temple. Try revisiting the temple and talking to everyone to ensure you've completed all necessary tasks to receive the drachma.
On the island
The one drachma coin is available at the Temple of Hades. Talk to the worker there, who will pay you to remove the graffiti from the outside walls. (You must take off every speck.)
Mythology island, on a statue in the museum
Find him and click on him.
You have to get it from a "many headed" dragon
You can find her when you first enter Mount Olympus with Hercules.