Instructions
Earn at least 700,000 bells. This can be done using any method in Animal Crossing: City Folk. The most profitable tasks will depend on which silver or gold tools you have unlocked at this point.
Go to the Civic Center in the city. A menu will appear with different options.
Select the ̶0;Town Fund̶1; option. Donate 200,000 bells.
Wait until a new bridge is built. This city upgrade must be built before you can earn the fountain.
Return to the Civic Center and select the ̶0;Town Fund̶1; option again. Donate 500,000 bells.
Wait until the fountain is built in the city. You can now get to the fountain anytime you wish when you visit the city.