The Bombchu Bag is an item in Phantom Hourglass. In order for Link to carry or buy Bombchus, he must obtain the Bombchu Bag from the Goron Temple. When Link obtains the bag, it will come with ten Bombchus.
The first upgrade for the Bombchu Bag can be purchased at the Goron Island Shop for 1000 Rupees. It will allow Link carry a maximum of twenty bombchus.
The last upgrade for the Bombchu Bag can be obtained on Dee Ess Island after completing the Goron Game. It will allow Link to carry a maximum of thirty bombchus.