The Adult Wallet can be obtained after defeating the giant Twilight Bug at Lake Hylia. Go to Hyrule Castle Town as a human, then go to the southeast alleyway and look for a door. Go inside and give Agatha any Golden Bug you have caught and she will give you an Adult Wallet (Big Wallet), capable of holding 600 Rupees. One Golden Bug is outside the south entrance of Castle Town, on the top of a large vine covered column. Blow up the rocks with a bomb and a Golden Ladybug will come out. It will resemble a point of light. Hit it with the boomerang and grab it with A. There are also two Golden Ants in Kakariko and Kakariko graveyard.

To get the Giant Wallet, give Agatha all 24 Golden Bugs.