The Adult's Wallet is obtained by collecting 10 Gold Skulltula Tokens and seeing the first freed brother of the House of Skulltula. He gives you the Adult's Wallet without having to pay anything as a reward for freeing him. This wallet is needed for a couple of small things, one of them being for purchasing the last Magic Bean that the Bean Seller sells Link. The Adult's Wallet is very practical in many circumstances even when it is not needed.