Keep hitting rocks you come across with your axe or shovel until you find a special one that gives a bag of Bells. Keep hitting the rock until the bags stop appearing. Note: This happens no more than once per day.
Take out your shovel and hit rocks with it until you find one that spits out Bells. Some of these can go up to 20,000 Bells. If you want to get as many Bells as possible, dig three holes away from the rock before hitting it. This will keep you in front of the rock because of the holes and thus you can hit it more times. Note: Do not put holes everywhere or the money bags will not appear.