Yeah, have you captured any bugs? You just need one.
If not here's and easy one: Go back to Kakariko Village and go into a empty house with a hole in the roof (but the front door does open). It's on the left side. Anyways, smash a box and a golden bug is under it. Go to Castle Town, enter on the East side, and before entering the center portion of castle town, head down. Keep going through door until you find a creepy place with a young girl. She'll be all like, "Blah blah blah, gimme some bugz". Give her the one, and she'll give you a wallet that stores 600 Rupees.
If you give her all 30 (or 20?) bugs, you'll then get a wallet that holds 1,000 rupees.