Here's how you can buy an unlocked iPhone 5 in the United States.
1. Go to the iPhone 5 page in the Apple Store.
2. Select the color of your iPhone 5 (black or white).
3. Click the "Or get iPhone unlocked and contract-free" link (just under the Choose a Network Carrier option).
4. Choose the storage size for your unlocked iPhone (16GB, 32GB or 64GB).
5. Click the green Continue button in the Summary box on the right side of your screen to finish buying your unlocked iPhone 5.