Look them up and buy them from the iPhone within the App Store app. They are only free for iOS devices purchased last September or later. If you have the same iTunes account on an older iOS device or iTunes on a computer then they will show up with their standard prices.
My ex-GF made the mistake several months ago of buying/downloading Pages on her iPad because she had gotten a new iPhone 5c and thought it would be free, but it cost her $9.99 because she did the purchase on the wrong device.