The Adult Wallet can only be obtained after defeating the giant Twilight Bug at Lake Hylia. Head to Hyrule Castle Town in human form and go to the southeast alley. Look for a door that leads to where a girl that collects bugs is. Give her any Golden Bug you have caught to receive an Adult Wallet, which can hold 600 rupees. An easy Golden Bug is right outside the south entrance of Castle Town, on a large column covered in vines. Blow up the rocks with a bomb to make a Golden Ladybug appear. Hit it with your boomerang and grab it with A. You can also find two Golden Ants in Kakariko Village and the graveyard there.