Have you upgraded your arrows to its maximum capacity? That's a good way to get free arrows, because every time you complete a "tier" of upgrade, it automatically fills up your arrows to the max.
To upgrade your arrow capacity, go to the Pond of Happiness, which is in the middle of Lake Hylia. You will need the Zora Flippers to get there because you need to swim to the island. For every 100 Rupees you throw into the pond, a fairy will appear and upgrade either your arrows or bombs.